This family consists of mammalian MRVI1 proteins which are related to the lymphoid-restricted membrane protein (JAW1) and the IP3 receptor associated cGMP kinase substrates A and B (IRAGA and IRAGB). The function of MRVI1 is unknown although mutations in the Mrvi1 gene induces myeloid leukaemia by altering the expression of a gene important for myeloid cell growth and/or differentiation so it has been speculated that Mrvi1 is a tumor suppressor gene. IRAG is very similar in sequence to MRVI1 and is an essential NO/cGKI-dependent regulator of IP3-induced calcium release. Activation of cGKI decreases IP3-stimulated elevations in intracellular calcium, induces smooth muscle relaxation and contributes to the antiproliferative and pro-apoptotic effects of NO/cGMP. Jaw1 is a member of a class of proteins with COOH-terminal hydrophobic membrane anchors and is structurally similar to proteins involved in vesicle targeting and fusion. This suggests that the function and/or the structure of the ER in lymphocytes may be modified by lymphoid-restricted resident ER proteins.