Spinal muscular atrophy (SMA), one of the most common fatal autosomal recessive diseases, is characterized by degeneration of motor neurons and muscular atrophy. The SMA disease gene, termed Survival of Motor Neurons (SMN), is deleted or mutated in over 98% of SMA patients. The function of the SMN protein is unknown. We found that SMN is tightly associated with a novel protein, SIP1, and together they form a specific complex with several spliceosomal snRNP proteins. SMN interacts directly with several of the snRNP Sm core proteins, including B, D1-3, and E. Interestingly, SIP1 has significant sequence similarity with Brr1, a yeast protein critical for snRNP biogenesis. These findings suggest a role for SMN and SIP1 in spliceosomal snRNP biogenesis and function and provide a likely molecular mechanism for the cause of SMA.