Joseph Burchenal, at Memorial Sloan-Kettering Cancer Center in New York, with Farber's help, started his own methotrexate study and found the same effects. He then decided to try to develop anti-metabolites in the same way as Farber, by making small changes in a metabolite needed by a cell to divide. With the help of George Hitchings and Gertrude Elion, two pharmaceutical chemists who were working at the Burroughs Wellcome Co. in Tuckahoe, many purine analogues were tested, culminating in the discovery of 6-mercaptopurine (6-MP), which was subsequently shown to be a highly active antileukemic drug.