We have been studying the heat-sensitive bimD6 mutation of Aspergillus nidulans. At a restrictive temperature, the chromosomes of bimD6 mutant strains fail to attach properly to the spindle microtubules, and the mutant also displays a high rate of chromosome loss. We previously cloned the sudA gene, an extragenic suppressor of the heat-sensitive bimD6 mutation and showed that it coded for a DA-box or SMC protein. SMC proteins have been demonstrated to function in chromosome condensation, segregation and global gene regulation. We have now cloned the sudD gene, another of the extragenic suppressor genes of the bimD6 mutation. The predicted SUDD protein is the founding member of a widely expressed protein family. Similar proteins are found in sequence databases for Saccharomyces cerevisiae, Caenorhabditis elegans, mammals and four species of archaebacteria. We have also cloned and sequenced a human cDNA that encodes the human homologue of SUDD and mapped the gene to 18q11.2. The predicted SUDD proteins from A. nidulans, Homo sapiens and S. cerevisiae all share a variety of features. The predicted proteins are approximately 60000Da in mass and have a serine-plus-threonine content of about 11%. The evolutionary conservation of the proteins suggests an ancient origin and conserved function for these proteins.