Centrin is a ubiquitous protein component of centrosomes and mitotic spindle poles. cDNA clones encoding centrin have been identified from vertebrate sources, and their sequences demonstrate that centrin is a highly conserved member of the EF-hand superfamily of calcium-binding proteins. Analysis of yeast and Chlamydomonas centrin mutants and experimental studies suggest that centrin has an essential role in the duplication of the centrosomes during the cell cycle, and in microtubule severing.