Bni1p, implicated in cell polarity control and microtubule regulation during yeast budding, is the Saccharomyces cerevisiae homolog of human Formin-homology proteins, such as FMN1, FMN2, FHOD1, FHOD3, FHDC1, GRID2IP, FMNL1, FMNL2, FMNL3, DIAPH1, DIAPH2, DIAPH3, DAAM1 and DAAM2. Cdc50p is necessary for subcellular localization of Bni1p and asymmetrical cell division. Lem3p and Ynr048wp are yeast homologs of Cdc50p; however, mammalian homologs of Cdc50p remained to be identified. Here, we identified and characterized CDC50A (TMEM30A), CDC50B (TMEM30B) and CDC50C (TMEM30C) genes by using bioinformatics. C6orf67 and FLJ33850 were representative human CDC50A and CDC50B cDNAs, respectively. Complete coding sequence of CDC50C cDNA was determined by assembling seven exons within AC129803.3 genome sequence. CDC50A, CDC50B and CDC50C genes were mapped to human chromosome 6q14.1, 14q23.1 and 3q12, respectively. Human CDC50A mRNA was expressed in embryonic stem (ES) cells, placenta, brain and chondrosarcoma, while CDC50B mRNA was expressed in pancreatic islet, kidney, prostate as well as in lung carcinoid, parathyroid tumor, bladder tumor, meningioma and pancreatic cancer. Mouse Cdc50a (2010200I23), Cdc50b (9130011B11) and Cdc50c (4933401B01) cDNAs were also identified. Mammalian CDC50 homologs, including human CDC50A (361 aa), CDC50B (351 aa), CDC50C (341 aa), mouse Cdc50a (364 aa), Cdc50b (353 aa) and Cdc50c (342 aa), were two-transmembrane-spanning proteins with one extracellular loop. Membrane topology and extracellular loop containing three Cys residues and one Asn-linked glycosylation site were evolutionarily conserved among mammalian CDC50 homologs and yeast Cdc50p homologs. Mammalian CDC50 homologs were predicted components of phospholipid-translocators just like yeast Cdc50p and Lem3p.