Could be involved in oogenesis. Involved in the regulation of endosome dynamics. Implicated in a novel signal transduction pathway, in which isoform 3 and CSK are sequentially activated by RHOD to regulate the motility of early endosomes through interactions with the actin cytoskeleton. Isoform 3 interacts with RHOD in the GTP-bound form. Isoform 3 is cytosolic but when coexpressed with RHOD, the 2 proteins colocalize to early endosomes. Expressed in testis, ovary, small intestine, prostate, lung, liver, kidney and leukocytes. Expressed from E16 in ovary and testis and during P6-P16 during differentiation of ovarian follicles. The DAD domain regulates activation via by an autoinhibitory interaction with the GBD/FH3 domain. This autoinhibition is released upon competitive binding of an activated GTPase. The release of DAD allows the FH2 domain to then nucleate and elongate nonbranched actin filaments (By similarity). The disease is caused by variants affecting the gene represented in this entry. Belongs to the formin homology family. Diaphanous subfamily.