Cyclin dependent kinase inhibitors (CdkIs) inhibit progression through the cell cycle by binding to cyclins, Cdks, or cyclin-Cdk complexes. CdkIs are classified into two groups based on protein structure. p57[Kip2] belongs to a group that also includes p27[Kip1] and p21 (also known as Sdi1, Cip1, Waf1 and Pic1). Members of this group have a homologous amino-terminal Cdk inhibitory domain. Members of the second group [p16 (INK4A), p15 (INK4B), p18 (INK4C), and p19 (INK4D)] contain ankyrin repeat motifs. p57[Kip2] is a potent, tight-binding inhibitor of several G1 and S phase cyclin-Cdk complexes including cyclin E-Cdk2, cyclin D2-Cdk4, and cyclin A-Cdk2. It inhibits the mitotic cyclin B1-Cdk1 complex to a lesser extent. mRNA studies suggest that p57[Kip2] expression is tissue-specific, the highest levels have been found in embryonic and adult skeletal muscle, heart, kidney, lung, eye and brain. This is in contrast to the widespread tissue expression of p27[Kip1] and p21 mRNA. The expression of p57[Kip2] to primarily terminally differentiated cells suggests that p57[Kip2] may play a specialized role in cell cycle control. Clone A120-1 reacts with human p57[Kip2]. Recombinant human p57[Kip2] was used as immunogen.