Formin homology proteins with FH1 and FH2 domains are signaling effectors for assembly and polarization of actin filaments. FH1 is the binding domain for Profilin, SRC, EMS1/Cortactin, FNBP1, FNBP2, FNBP3, FNBP4 and WBP4/Fbp21, while FH2 is the actin-filament modification domain. Here, we identified and characterized a novel member of Formin-homology gene family, Diaphanous homology 3 (DIAPH3), by using bioinformatics. DIAPH3 isoform 1, corresponding to 3'-truncated FLJ34705 cDNA and 5'-divergent IMAGE5265490 cDNA, encodes full-length DIAPH3 protein (1112 aa), while DIAPH3 isoform 2, identical to NM_030932.2 cDNA, encodes N-terminally truncated DIAPH3 protein (849 aa). DIAPH3 isoform 1, consisting of exons 1-27, was expressed in lymph node, erythroid progenitor cells as well as in pancreatic cancer. DIAPH3 isoform 2, consisting of exons 1b and 8-27, was expressed in testis. DIAPH3 gene at human chromosome 13q21.2 was found to encode two isoforms due to alternative splicing of the alternative promoter type. Full-length human DIAPH3 protein, consisting of FDD, FH1 and FH2 domains, showed 51.3% total-amino-acid identity with DIAPH1, and 57.3% total-amino-acid identity with DIAPH2. FMNL1/FMNL, FMNL2/FHOD2, FMNL3/WBP3, DAAM1, DAAM2, DIAPH1, DIAPH2 and DIAPH3 were classified as the FDD-type Formin homology proteins, while GRID2IP/Delphilin, FHOD1, Fmn1 and Fmn2 were classified as the non-FDD-type Formin homology proteins. This is the first report on identification and characterization of human DIAPH3 gene.