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Eukaryotic mismatch repair of simple base mispairs and small insertion-deletion loops is activated by the binding of a heterodimeric complex composed of MutS homolog 2(MSH2) and MSH6. Here we report the cloning of zebrafish (Danio rerio) MSH2 (zMSH2) cDNA that has an open reading frame of 2811 nucleotides encoding a polypeptide of 936 amino acids. The deduced amino acid sequence of zMSH2 shares a 69% identity to both human and mouse MSH2. The zMSH2 protein contains a putative tyrosine-42 mismatch-contacting residue located at the N-terminal mismatch recognition region and four C-terminal ATP-binding consensus sequences conserved among MutS homologs. The 105-kDa recombinant zMSH2 bound apparently stronger to a G–T heteroduplex than to a homoduplex probe as shown by a gel shift assay. A preferential expression of both zMSH2 and zMSH6 mRNA in early embryos was found by Northern blot analysis. Whole mount in situ hybridization revealed a major expression of zMSH2 in different regions of the brain, including eyes, telencephalon, and the fourth ventricle in 12- to 48-h-old embryos. The production of zMSH2 mRNA gradually decreased in more mature 60- to 120-h-old zebrafish, reflecting a positive correlation between the amount of proliferating cells and MSH gene expression. [Copyright &y& Elsevier]