6-Mercaptopurine is an analog of hypoxanthine, an intermediate in purine nucleotide biosynthesis. When mercaptopurine is made into a nucleotide by a cell, it stops DNA replication from occurring because it is incorporated into DNA by DNA polymerase instead of the proper nucleotide.

6-Mercaptopurine is an anticancer medication. It inhibits the uncontrolled DNA replication associated with proliferation of white blood cells in leukemia.