The PITSLRE beta1 protein, a distantly related member of the Cdk family of protein kinases, induces apoptosis after low levels of ectopic expression. Apoptosis, or programmed cell death, is similarly induced by ectopic expression of an amino terminal deletion mutant retaining the catalytic and carboxy
terminal domains of PITSLRE beta1, but not by other mutants lacking Histone H1 kinase activity or by other Cdk family members. The terminology for the ten isoforms of the PITSLRE subfamily of proteins is based on the conserved PSTAIRE box region of Cdc2 p34. Depending on which of the PITSLRE genes produce the protein, the cDNA and protein are designated alpha, beta or gamma (i.e., PITSLRE A gene, alpha; PITSLRE B gene, beta and PITSLRE C gene, gamma ). Some of the isoforms such as PITSLRE alpha1 (T cells) and PITSLRE beta1 (B cells and brain), are expressed in specific cell types, while others are expressed ubiquitously.