AMSH, a molecule that associates with STAM1, is involved in the in vitro cell growth signaling mediated by interleukin 2 and granulocyte-macrophage colony-stimulating factor. To investigate the in vivo functional role of AMSH, we have generated AMSH-deficient mice by gene targeting. The AMSH-deficient mice were morphologically indistinguishable from their littermates at birth, and histopathological examinations revealed normal morphogenesis in all tissues tested. However, all the AMSH-deficient mice exhibited postnatal growth retardation and died between postnatal day 19 (P19) and P23. Examination of brain sections at P6 demonstrated significant loss of neurons and apoptotic cells in the CA1 subfield of the hippocampus. Brain atrophy developed by P16 and was accompanied by complete loss of the CA1 neurons in the hippocampus and marked atrophy of the cerebral cortex. Furthermore, AMSH-deficient hippocampal neuronal cells were unable to survive in vitro, even in the presence of several stimulatory cytokines, while AMSH-deficient cerebellar neurons, thymocytes, and embryonic fibroblasts survived normally. Taken together, these observations indicate that AMSH is an essential molecule for the survival of neuronal cells in early postnatal mice.Cytokines are essential factors for cell activation, differentiation, survival, and apoptosis in many biological events. The effects of cytokines on target cells are mediated by their interactions with specific receptors, which transduce the intracellular signals in the target cells. Among the many cytokines, interleukin 2 (IL-2) is known to be a critical soluble ligand for activating T cells in immune responses (12,34,35). During the search for signaling molecules involved in the IL-2-mediated signaling pathway, we identified the STAM family of molecules, STAM1 and STAM2, both of which are associated with Jak2 and Jak3 tyrosine kinases (9,36,37). We subsequently isolated a novel adapter molecule from human T cells that we named AMSH, for associated molecule with the SH3 domain of STAM1 (39). Recently, a novel SH3-binding motif (SBM) of AMSH that interacts with the SH3 domain of both STAM1 and STAM2 was identified (17, 39). SH3 deletion mutants of the STAMs act as dominant-negative forms in signaling that induces cell growth, and the wild-type, but not the mutant, STAMs enhance c-myc induction that is mediated by IL-2 and granulocyte-macrophage colony-stimulating factor (GM-CSF) (37). Hence, we hypothesized that AMSH might be involved in the cytokine-mediated signaling through its interaction with the STAMs. In this context, we have already demonstrated that an AMSH mutant with its C-terminal half deleted and retaining its STAM1-binding ability confers a dominant-negative effect on IL-2-and GM-CSF-mediated signaling pathways that induce DNA synthesis and c-myc transcription (39). The deleted C terminus of AMSH contained an Mov34/MPN domain (3), which is conserved in several subunits of the COP9 signalsome, although the function of this domain is still unknown. Collectively, thes...