AT1521 was an antiretroviral drug developed to drastically delay the progression of t-Virus within infected bodies. AT1521 had a high success rate during mice trials, with infection in 99.91% of mice being halted. Success of the treatment however, was limited only to people who had not yet shown symptoms.
AT1521 was developed during the lead-up to the Raccoon City Destruction Incident, and appears to have been developed at the Umbrella R&D Center in 1998 as part of a team led by Dr. Michael Jenkins and Dr. Linda Baldwin. Experimental antiretroviral drugs were available outside the facility by mid-1998, as U.B.C.S. mercenaries and the P-12A plant employees possessed them. Whether these were also AT1521 or a parallel development is uncertain. Dr. Linda Baldwin is the only known member of the team to have escaped the city and, though her equipment was destroyed along with any existing samples, she was able to recreate it from memory.