The p51/p63 gene, a novel member of the p53 gene family, has recently been identified at 3q27-9. There are at least six major isotypes of p51/p63 mRNA transcripts. p51A/TAp63gamma has the potential to induce apoptosis and growth suppression in a manner similar to p53, and other isotypes may suppress the p53 and p51A1TAp63gamma genes in a dominant-negative manner. We analyzed the mutation and expression of the p51/p63 gene in 80 cases of chronic myelogenous leukemia (CML) to evaluate its role in blastic transformation. Expression of the p51/p63 gene was detected in 74 cases. The alpha isotype of p51/p63 transcripts was dominantly expressed in 72 of these 74 cases. There was no correlation between the isotypes of p51/p63 transcripts and the clinical phase. Mutations of the p51/p63 gene were found in six cases. All these mutated cases expressed p51B/TAp63 alpha. In four of the six cases, the mutations were within a limited region (codon 151-170) corresponding to the DNA-binding domain. We hypothesized that this limited region is a hot spot for mutation of the p51/p63 gene. Mutations of the p53 gene were found in four cases of CML in blastic crisis (BC). Frequencies of the p51/p63 and p53 gene mutations were higher in BC (p51/p63 gene, 11.8%; p53 gene, 7.8%) than in the chronic phase (p51/p63 gene, 1.5%; p53 gene, 0%). The p51/p63 gene mutation may act similarly to the p53 gene mutation as a genetic alteration potentially responsible for the progression of CML.