Constitutive activation of BCR-ABL tyrosine kinase is the hallmark of CML in 95% of patients. The reciprocal fusion product, ABL-BCR, is deleted in 20% of patients. Studies have revealed a 120 kb deletion centromeric of ABL encompassing: PRDM12, a putative histone methyltransferase, and EXOSC2, a 3'-5' exoribonuclease.PRDM12 is one of 16 PR family members. PRDM12 consists of a PR domain and 3 zinc fingers. PR domains are thought to function as histone methyltransferases (HMT) as they share sequence similarity to the SET domain, known histone methyltransferases. The zinc fingers are important in DNA binding and protein-protein interactions. The PR domain of PRDM12 does not possess intrinsic HMT activity. Interestingly, PRDM12 is found associated with chromatin, suggesting it may be important in recruiting a complex of proteins involved in repression of transcription, as does another PR family member, PRDM1/PRDI-BF1/BLIMP-1. The involvement of PRDM12 with chromatin suggests a possible role in transcription regulation. This may reveal its importance in a more aggressive form of CML.