Cyclin-dependent kinases (CDKs) are protein kinases involved in critical cellular processes, such as cell cycle or transcription, whose activity requires association with specific cyclin subunits. Based on sequence similarity, the human genome contains 21 genes encoding CDKs and five additional genes encoding a more distant group of proteins known as CDK-like (CDKL) kinases. The current nomenclature for CDK proteins includes 11 classical CDKs (CDK1–11), two newly proposed family members (CDK12 and 13) and additional proteins whose names are based on the presence of a cyclin-binding element (PFTAIRE and PCTAIRE proteins) or simply based on a sequence relationship with the original CDKs, such as CDC2-like kinases (CDC2L) or cell cycle-related kinases (CCRK) 1. Here we propose the use of'CDK'for all CDK family members (CDK1–20) on the basis of similarities in sequence and function as described below. This nomenclature will facilitate the rational and comparative study of the poorly understood family members and the analysis of their relevance to human disease.

During the Cold Spring Harbor Symposium on the Cell Cycle in 1991, a group of interested scientists proposed that members of this kinase family should be called cyclin-dependent kinases (CDKs). The consensus was that no kinase should be called a'CDK'until it was proven rigorously that its activity depended on association with some cyclin-like regulatory subunit. Known family members were then renamed CDK1–6, and further members (CDK7–CDK10) were cloned and characterized in the following years 1. CDK11 has been used to refer to the protein encoded by three different human loci (CDC2L1, CDC2L2 and CDC2L6) in different publications. CDC2L1 and CDC2L2 are two highly similar human genes, which originated by duplication in human chromosome 1, and they encode almost identical protein kinases. Each of these loci encodes at least two major peptides: a protein kinase of relative molecular mass 110,000 (M r 110K), and a smaller 58K isoform that is expressed following alternative initiation of translation. To make the situation more complex, some publications, including the original description of the human kinome 2, refer to CDK11 as the protein encoded by CDC2L6 (Supplementary Information, Table S1). The single Cdc2l1 gene in the mouse genome encodes a protein more similar to human CDC2L2 (91% identity) than CDC2L1 (87%). We therefore propose the use of Cdk11 for the mouse gene and CDK11A and CDK11B for the human CDC2L2 and CDC2L1 loci, respectively (Fig. 1; Supplementary Information, Table S1). The corresponding human proteins should be referred to as CDK11A p58 or CDK11A p110 for the CDC2L2-encoded proteins, and CDK11B p58 or CDK11B p110 for the CDC2L1-encoded proteins. Two additional kinases, formerly known as Crk7 (CrkRS) and Ched (Cdc2L5), were recently renamed CDK12 and CDK13, as they were reported to interact with cyclin L1 and cyclin L2 (Refs 3, 4).