Abstract A novel member of the Formin/Diaphanous family of proteins was cloned and characterized. A 4 kB mRNA is ubiquitously expressed but is found in abundance in the spleen. FHOS (Formin Homologue Overexpressed in Spleen) contains a 3414 bp open reading frame and encodes for an approximately 128 kDa protein. FHOS has sequence homology to Diaphanous and Formin proteins within the Formin Homology (FH)1 and FH2 domains. FHOS also contains a coiled-coil, a collagen-like domain, two nuclear localization signals, and several potential PKC and PKA phosphorylation sites. FHOS-specific antiserum was generated and used to determine that FHOS is a predominantly cytoplasmic protein and is expressed in a variety of human cell lines. FHOS was mapped to chromosome 16q22 between framework markers WI-5594 and WI-9392.