Yawn suffered mutations atypical of other adders, having grown rapidly to forty meters in length due to excess hormone production. It possessed amphibian characteristics,[excerpt 1] with mucus secretions covering the skin to aid in injury recovery rather than having dry, rough skin.[excerpt 2] The size of Yawn's mouth and its oesophagus made it easily capable of swallowing a human, whole.[excerpt 3]
The most noticeable aspect of its t-Virus infection was its enormous length and girth. It also proved to retain its predator intellect, easily able to stalk and attack its prey before they even realised it. However, the Yawn exhibited examples of uncoordinated mutations, as its size became greatly inconsistent with its length, and its body became covered with various sores and tumours.
The potency of its venom was also significantly enhanced, requiring a specially-formulated serum to counteract it. Its venom could quickly immobilise a human being in minutes, and would kill the victim shortly afterwards if left untreated.