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DYSREGULATION of apoptosis can result in inappropriate suppression of cell death, as occurs in the development of some cancers¹, or in failure to control the extent of cell death, as is believed to occur in acquired immunodeficiency and certain neurodegenera-tive disorders, such as spinal muscular atrophy (SMA). Recently, we isolated a candidate gene, encoding neuronal apoptosis inhibitor protein (NAIP)², for SMA. This gene is homologous to two baculovirus inhibitor of apoptosis proteins^3,4(Cp-IAP and Op-IAP) and is partly deleted in individuals with type I SMA. A second SMA candidate gene encoding survival motor neuron (SMN), which is contiguous with the NAIP locus on 5ql3.1, was also reported5. Here we demonstrate a NAIP-mediated inhibition of apoptosis induced by a variety of signals, and have identified three additional human complementary DNAs and a Drosophila melanogaster sequence that …