FNBP1 and FNBP2 are SH3-type Formin-binding proteins. FNBP1 consists of FCH, FBH, HR1 and SH3 domains, while FNBP2 consists of FCH, FBH, RhoGAP and SH3 domains. Here, we identified novel genes FCHSD1 and FCHSD2, which were distantly related to FNBP1 and FNBP2. FCHSD1 and FCHSD2 genes with conserved exon-intron structure were located at human chromosome 5q31.3 and 11q13.4, respectively. Complete coding sequence of human FCHSD1 was derived from FLJ00007 (NM_033449.1) cDNA. KIAA0769 (NM_014824.1), encoding N-terminally truncated 684-aa protein, was an aberrant human FCHSD2 cDNA with a frame shift due to skipping of 98-bp exon 2. Complete coding sequence of human FCHSD2 cDNA was determined by assembling CF995054 EST and KIAA0769 cDNA. A030002D08Rik (NM_175684.3) was the representative mouse Fchsd1 cDNA, while BC034086 (NM_199012.1) was a variant mouse Fchsd2 cDNA with an insertion of 72-bp additional exon. CG4684 was the Drosophila homolog of mammalian FCHSD family genes. Human FCHSD1 (690 aa) showed 41.7% total-amino-acid identity with human FCHSD2 (740 aa), and 91.0% total-amino-acid identity with mouse Fchsd1. Human FCHSD2 showed 96.5% total-amino-acid identity with mouse Fchsd2. Mammalian FCHSD family proteins shared the common domain structure consisting of FCH, FBH, two SH3 and C-terminal Proline-rich domains. FCHSD family proteins (FCHSD1 and FCHSD2), FNBP1 family proteins (FNBP1, FNBP1L and TRIP10/CIP4) and FNBP2 family proteins (FNBP2, ARHGAP13/SRGAP1, ARHGAP14/SRGAP2 and ARHGAP4) were found constituting the FCFBS superfamily characterized by FCH, FBH and SH3 domains. This is the first report on identification and characterization of the FCHSD family genes.

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