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Resident Evil Wiki
Further notes


Dr. Birkin's transformation saw him go through a number of mutant forms. There were five distinct stages of his evolution, though these included intermediate stages as well. They are dubbed "G1" through "G5".


Dr. Birkin's initial mutation kept much of his body unchanged, with the mutations only being seen on the right side of the body, with the arm, torso and face being affected as it was on this side of the body he was shot. In Resident Evil 2 original and The Darkside Chronicles, the mutation made William's right side of his body grotesquely increase in muscle mass, making his skin tear up and take on a turn a sickly reddish hue, while in most of his shoulder arises a large blood-red eyeball beneath a large bone-like protrusion exposed. In the remake version of Resident Evil 2, the mutated musculature on its right side is much more pronounced and there are more protrusions similar to bones near the eyeball. In the three games, the left side of the body is more recognisably human. A side-effect of maintaining a human brain was occasional bouts of dizziness.[1][excerpt 1] The G1 form appeared in two stages, distinguished by the appearance of claws on the mutated right hand, which were used on 23 September to kill a number of USS operators. By the night of 29 September Dr. Birkin had evolved to no longer require them, instead having an enlarged but otherwise human hand capable of handling weapons.

Interestingly, G1's right arm periodically "de-evolves" slightly, becoming smaller as the eye on the shoulder closes and the spikes retract; this seems to represent Birkin attempting to restrain the G-Virus's degenerative effects on his mind, although it's clearly a losing battle. This is taken further in the Resident Evil 2 remake, where Birkin seems to retain a degree of his humanity; when his right arm shrinks, his cries become less monstrous and distorted, he screams "Sherry!", "Help Me!" and "Kill!", and he occasionally pauses to swing at thin air or attempt to pry open the eyeball on his shoulder, leaving him vulnerable. In cutscenes, he also appears to be attempting to protect Sherry by attacking threats to her safety (such as Brian Irons and the T-00) before the G-Virus instincts take over once again.


The G2 form experienced the growth of a new head and brain to replace the original, the face of which descended to the torso's left side. The torso in general sees an increase in size, becoming wider as not only the skeleton and muscles are changed but two new arms begin growing asymmetrically under both shoulders. The legs also become thicker, with the tibula and fibia increasing in length. The mutant right arm also went through further changes, regrowing the claws it once had early in the G1 stage while shoulder eyeball grows considerably.[2][excerpt 2] In The Darkside Chronicles, the most of the skin of the G2 has a sickly pale greenish tone. In the remake version of Resident Evil 2, the mutated musculature remains a sickly reddish hue, but is even more pronounced, being that its left secondary arm is more functional. The right side of its stomach shows either signs of the other arm developing or part of its rib cage, its claws are longer, and the developing rings of spines on its chest have been removed. Additionally, the developing secondary cranium has yellow eyes (bloody parts of flesh slightly obscuring the right eye) and a mouth to show its fangs.

Interestingly, this form of G managed to recognize Annette, to some extent at least. When it found her, it hesitated and looked at her for a few moments, but attacked her when she drew a gun on it, recognizing her as a threat.


The G3 form saw the increased symetricalisation of the body. The legs became black and dark grey coloured, with the growth of an eye on the left thigh. The two new arms grow further and became the same size, and grew their own shoulders to descend from. The original two arms were pushed backwards and kept their own shoulders, which allowed for the proper use of four arms. The vestigial human face shrunk in size and positioned itself against the front left shoulder. The G3 form also continued the changing movement of the heart; positioned near the shoulder in the G2 form, it moved to the middle of the upper torso. This form has been noted for its demonic appearance.[3][excerpt 3] In The Darkside Chronicles, the most of the skin of the G3 has a black and dark grey coloured tone, while in Resident Evil 2 remake, G3 has a slightly more hunched physique and an elongated neck, giving his head a slight resemblance to his later forms in the original game, being that his skin remains sickly reddish hue but his original left arm gets even bigger than his right. Furthermore, the remnants of Birkin's human face are horribly distorted, barely visible under G's lower-left arm, the eye on its left thigh has become proportionately larger, and the monster has several more bony protrusions on its limbs while the "mouth" on chest is slightly smaller and less toothy; however, it's revealed to contain a cluster of small eyeballs that are exposed when the monster is significantly damaged.


Following damage sustained in his G3 form, Dr. Birkin mutated to the G4 form. This form saw a radical change in gait, going from bipedal to quadrupedal as the two largest arms became legs. G4 saw an enhanced mobility surpassing the G3 form in spite of losing its high-jumping skills. A large mouth has formed out from his chest out of ribs that previously protected the heart. The need for such a large mouth was so the creature could satisfy its increasing demands for energy.[4][excerpt 4] Notably the vestigial face has all but disappeared into the shoulder of the smaller pair of arms. In the remake, the G4 shape mutation changes completely, resembling a heavily degraded version of G3 on its way to mutating into G5. Its secondary arms have vanished, and the "mouth" on its chest now dominates most of its torso, with large tusks and eyes protruding in an almost random fashion. Its flesh has become bloated and almost tumorous - seemingly dissolving into independent tendrils only partially fused together - but the monster maintains a bipedal stance, dropping to all fours to move more quickly and climb up walls. The eyes of its degraded head are green, while the rest dotting its body are their usual yellow-to-red. The second head also became a little more natural and also fused with the torso while the upper jaw has several small teeth.


The G5 form was a further degradation of the G4 form. The mouth and digestive system of G4 remained to support the massive nutrient requirements. Whole bodies were consumed by G5, which could eat even Zombies. However, these were digested over time, and partially consumed and recognisably-human biomass covered its surface. The body itself became more mollusk-like in appearance, though propelling by using four tentacles to drag itself along.[5][excerpt 5] In The Darkside Chronicles, the mutated form of the G5 vaguely resembles an arthropod with two large red eyes while in its right tentacle also appears another red eye. In the Resident Evil 2 remake, G5 is larger than previous appearances, it has several tumour-like eyes inside of its mouth, with the largest in its center, and also seems to spread tiny tendrils of biomass ahead of its main mass. The huge eye in its jaw had a blue iris.

This form also has some level of intelligence, since it realized the train car it was on was about to explode and screamed “SHERRY!”


After injecting himself with G-Virus to survive his critical bullet wounds, Birkin initially retained his human form; however, he rapidly began to mutate and develop inhuman abilities. The mutations in his transformations gave him a very grotesque muscle mass and bony claws, which, in turn, gave him superhuman strength able to easily crush anyone (in Resident Evil 2 remake, he killed the T-00 in a single blow). His increased muscle mass also gave him superhuman endurance that grew with each transformation he underwent. Birkin has also demonstrated the ability to implant G-Virus embryos in other people, with mixed results depending on their biology.

However, Birkin's true power is his resilience; because of the G-Virus in his body, he developed fierce regenerative abilities which make him biologically almost immortal and very difficult to defeat. Even though most of his body is eliminated, the virus forced Birkin to undergo more mutations in response, gaining stronger abilities and a more monstrous appearance. However, this regenerative ability had its limits; after taking catastrophic damage, Birkin was forced to mutate into a shapeless blob to persist. The eye-like growths around his body also proved more vulnerable than the rest of him; even steel pipe could damage them to the point of temporarily incapacitating Birkin. Finally, not even the G-Virus could save Birkin from having his entire body destroyed at once, allowing Claire and Leon to kill him for good when he was engulfed by the explosion and destruction of the NEST.


  1. Excerpt from Official Guide Book, p.53: "大きく右肩がせり出した左右非対称の姿は、 人間の原型をとどめているだけに、 その異様さが、 よりきわだって映る。 変形の初期段階である現時点でもゾンビとは比較にならない耐久力を有し、 G細胞に浸食された右腕は鉄柵を容易に引きちぎるほどの怪力を誇る。 ときおりめまいに似た不安な動きを見せるが、 これはG生物化によって身体の質量が大幅に増加していることに起因する。"
  2. Excerpt from Official Guide Book, p.54: "左腕をのぞいた上半身全域にG細胞の浸食が進み、 ベースとなった人間の頭とは別に脊椎付近から新しい頭部が造り出されている。 それにともない内部の骨格は大きく変貌しており、巨大な右腕のツメも、 手の先の骨が尖鋭化し、出し入れ可能になったものと推測される。 なお、 その姿から判断して、 Gの脳はまったく新しいものが形成されている模様。"
  3. Excerpt from Official Guide Book, p.55: "増殖したG細胞が下半身にまで到達し、 眼球状組織が新しく左大腿部にも発生した。 第2形態で生じつつあった補助腕が、 通常の腕と遜色ないまでに発達をとげ、 背中に大きく張り出した本来の腕は、 有翼の悪魔を連想させるシルエットを作り出している。 降起した胸部には巨大な心臓があると考えられ、 循環器系にも異変が起きていることがうかがえる。"
  4. Excerpt from Official Guide Book, p.56: "G細胞の増殖もともない進行なるGの形態変化は、生命危機に即応した超回復によっても引き起こされる。この段階では、 2足歩行をやめ獣型になることで、 人型では実現しえない高い運動性を獲得した。 頭部と一体をなす胸部の大アゴは、 食物摂取をより活発に行なうためのものと考えられ、 体格に比例してエネルギー需要が高まっていることを裏づけといる。"
  5. Excerpt from Official Guide Book, p.58: "第4形態で食物を大量摂取したことを反映して、 身体の質量・体積がかつてないほどに増大している。 4本あった腕は触手に変形し、 せまい場所にも器用に侵入する軟体動物のような身体を得るに至った。 体表を覆う未消化物質には人型の肉塊も見てとれ、 研究所のゾンビたちをエネルギー源として取りこんでいたことがわかる。"
  1. Tahara (ed.), Official Guide Book, p.53.
  2. Tahara (ed.), Official Guide Book, p.54.
  3. Tahara (ed.), Official Guide Book, p.55.
  4. Tahara (ed.), Official Guide Book, p.56.
  5. Tahara (ed.), Official Guide Book, p.58.