INTRODUCTION A fundamental question in biology is how cell division, growth and differentiation are coordinately regulated during development of multicellular organisms. This coordination likely involves connections between developmental signals and the cell-cycle machinery. Cyclin-dependent kinases (Cdks) have been identified as key components of the cell-cycle machinery in all eukaryotes studied (Forsburg and Nurse, 1991; Nigg, 1995; Norbury and Nurse, 1992). In yeasts, progression through the cell cycle is controlled by a single Cdk, encoded by the CDC28 and cdc2 + genes in Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively. The CDC28/cdc2 catalytic subunit requires association with a cyclin regulatory partner for kinase activity, and different cyclins are involved in progression through G 1 , S and M phase. Regulation of cell- cycle progression is more complex in multicellular organisms; a large family of kinases related to CDC28/cdc2 has been identified in metazoans and several of these kinases have been implicated in cell-cycle regulation (reviewed by Nigg, 1995). In addition, various cyclins are expressed during distinct phases of the cell cycle in metazoans. Thus, progression through the cell cycle in higher eukaryotes is thought to require successive activation of different Cdks, regulated in part by transient associations with distinct cyclins. Other levels of regulation include association with kinase inhibitors and activating as well as inactivating phosphorylations. When vertebrate cells enter a division cycle, the first CDKs to become activated are Cdk4 and/or Cdk6 in association with D-type cyclins (Sherr, 1994, 1996). Late in G 1 , Cdk2 is activated and can be found associated first with cyclin E and subsequently cyclin A (Tsai et al., 1993a). Kinase activity of Cdk1/Cdc2 in combination with A- and B-type cyclins peaks at the G 2 /M transition (Draetta and Beach, 1988; Tsai et al., 1993a). Several lines of evidence indicate that these Cdks regulate specific cell-cycle transitions. Extracellular signals, such as growth factors, influence expression levels of D-type cyclins and association with Cdk4 and Cdk6 (Matsushime et al., 1994, 1991). Ectopic expression of cyclin D, as well as cyclin E, shortens G 1 phase and accelerates entry into S phase (Ohtsubo and Roberts, 1993; Quelle et al., 1993). Inhibition of either cyclin A activity or Cdk2 activity inhibits DNA replication (Fang and Newport, 1991; Girard et al., 1991; Pagano et al., 1993; Tsai et al., 1993a; van den Heuvel and Harlow, 1993), while inhibition of Cdk1/Cdc2 prevents entry into mitosis (Fang and Newport, 1991; Riabowol et al., 1989; Th’ng et al., 1990; van den Heuvel and Harlow, 1993). The function of several other cdc2-related kinases is currently either unknown or, for Cdk5, appears unrelated to cell-cycle progression (Chae et al., 1997; Gilmore et al., 1998; Nikolic et 2227 Development 126, 2227-2239 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV6396 We have identified six protein kinases that belong to the family of cdc2-related kinases in Caenorhabditis elegans. Results from RNA interference experiments indicate that at least one of these kinases is required for cell-cycle progression during meiosis and mitosis. This kinase, encoded by the ncc-1 gene, is closely related to human Cdk1/Cdc2, Cdk2 and Cdk3 and yeast CDC28/cdc2 + . We addressed whether ncc-1 acts to promote passage through a single transition or multiple transitions in the cell cycle, analogous to Cdks in vertebrates or yeasts, respectively. We isolated five recessive ncc-1 mutations in a genetic screen for mutants that resemble larval arrested ncc-1(RNAi) animals. Our results indicate that maternal ncc-1 product is sufficient for embryogenesis, and that zygotic expression is required for cell divisions during larval development. Cells that form the postembryonic lineages in wild-type animals do not enter mitosis in ncc-1 mutants, as indicated by lack of chromosome condensation and nuclear envelope breakdown. However, progression through G 1 and S phase appears unaffected, as revealed by expression of ribonucleotide reductase, incorporation of BrdU and DNA quantitation. Our results indicate that C. elegans uses multiple Cdks to regulate cell-cycle transitions and that ncc-1 is the C. elegans ortholog of Cdk1/Cdc2 in other metazoans, required for M phase in meiotic as well as mitotic cell cycles. Key words: Caenorhabditis elegans, ncc-1, Cell cycle, Cdc2, Cdk, cdk5, PCTAIRE, Cell-division SUMMARY The Caenorhabditis elegans gene ncc-1 encodes a cdc2-related kinase required for M phase in meiotic and mitotic cell divisions, but not for S phase Mike Boxem, Dayalan G. Srinivasan and Sander van den Heuvel* Massachusetts General Hospital Cancer Center, Building 149, 13th Street, Charlestown, MA 02129, USA *Author for correspondence (e-mail: heuvel@helix.mgh.harvard.edu) Accepted 22 February; published on WWW 19 April 1999