A compound of the formula R1-Leu-A2-Val-A4-Ile-A6-Leu-A8-A9-A10-R2 SEQ ID NO: 1 or a pharmaceutically acceptable salt thereof, which disrupts the binding between the AF4 and AF9 proteins in mammalian cells in vitro and in vivo is useful in the treatment, prophylaxsis or diagnosis of various forms of leukemia. Such compounds are also useful in drug development for non-peptide mimics of the compounds of the invention.