The Doc2 (double C2) family consists of two isoforms (Doc2 alpha and Doc2 beta) characterized by an N-terminal Munc13-1 interacting domain (Mid) and two C2 domains that interact with Ca2+ and phospholipid at the C-terminus. This Ca2+-binding property is thought to be important to the regulation of neurotransmitter release. In this paper, we report a third isoform of mouse Doc2, named Doc2 gamma. Doc2 gamma also contains a putative Mid domain and two C2 domains, and it is 45.6 and 43.2% identical to mouse Doc2 alpha: and Doc2 beta, respectively, at the amino acid level. In contrast to the other Doca isoforms, the C2 domains of Doc2 gamma impair Ca2+-dependent phospholipid binding activity. The highest expression of Doc2 gamma mRNA was found in the heart, but occurs ubiquitously, the same as Doc2 beta. These findings indicate that Doc2 gamma may also function as an effector for Munc13-1 and that it may be involved in the regulation of vesicular trafficking.