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In mitotic division, sister chromatids are attached until their segregation in anaphase. This cohesion between sister chromatids is mediated by a multi-subunit complex called cohesin, which is composed of four proteins, namely Scc1/Rad21, Scc3, Smc1, and Smc3. The nematode Caenorhabditis elegans has four Scc1/Rad21 family members (COH-1, SCC-1/COH-2, COH-3, and REC-8). Of these, REC-8 was previously shown as the functional homolog of yeast Rec8, which is involved in chromosome cohesion during meiosis (Pasierbek et al., Gene Dev. [2001]. 15, 1349-1360). Now, a distinct developmental function of SCC-1/COH-2 and COH-1 has become apparent (Mito et al. Mol. Biol. Cell [2003]. in this issue). SCC-1/COH-2 is involved in proper chromosome segregation during mitosis—the timing and subcellular localization of SCC-1/COH-2 is strictly regulated in a cell-cycle-dependent manner throughout development. The cover photograph shows images of an L1 larva stained for SCC-1/COH-2 (anti-SCC-1/COH-2 antibody: top) and for DNA (SYTOX: middle), together with their superimposition (bottom). SCC-1/COH-2 is present only in the nucleus of cells that are about to divide (i.e., 22 P lineage cells to constitute the ventral nerve cord, 4 Q lineage cells to produce posterior neuronal cells, and proliferating germ cells). In contrast, COH-1 is present on the chromosomes in virtually all somatic cells, including nonmitotic cells, suggesting that it has a novel function necessary for developemnt but unrelated to mitosis. Thus far, COH-1 is the only Scc1/Rad21 family member that is implicated in a function unrelated to chromosomal cohesion. - Asako Sugimoto, Yoshiko Mito, Masayuki Yamamoto