AT1521 is an experimental agent to suppress the T-Virus, developed by, among others, Linda; Carter, virologists working for the Umbrella Corporation in their official facility in Raccoon City. The AT1521 is water-like in appearance.
Developed at some point prior to or during the 1998 outbreak that devastated Raccoon City and annihilated its population, AT1521 is a serum designed to inhibit the growth of the virus.
As the final night of the outbreak drew to a close, and Raccoon City's ultimate destruction grew near, the facility where the agent was stored came under siege from within. A rapidly spreading fire and escaped Hunters killed almost all of the staff. A desperate Linda returned to the lab to obtain a sample of the reagent and became trapped within the building with Carter and a group of survivors from the city. With the aid of the Tyrant R, they fought off the hunters and made it to the escape elevator.
But something unexpected happened. The Tyrant rebelled, killing Carter and destroying the only existing AT1521 sample. The desperate survivors were forced to dodge the Tyrant and other perils of the city, with an injured Linda in tow, before facing off against the B.O.W. Nyx and escaping the city seconds before it's annihilation.
Later, Linda was able to recreate the sample from memory, which proved crucial to ensuring that the T-Virus outbreak in Raccoon City did not spread any further.