The survival of motor neurons (SMN) gene is the disease gene of spinal muscular atrophy (SMA), a common motor neuron degenerative disease. The SMN protein is part of a complex containing several proteins, of which one, SIP1 (SMN interacting protein 1), has been characterized so far. The SMN complex is found in both the cytoplasm and in the nucleus, where it is concentrated in bodies called gems. In the cytoplasm, SMN and SIP1 interact with the Sm core proteins of spliceosomal small nuclear ribonucleoproteins (snRNPs), and they play a critical role in snRNP assembly. In the nucleus, SMN is required for pre-mRNA splicing, likely by serving in the regeneration of snRNPs. A DEAD box putative RNA helicase, named Gemin 3 which is another component of the SMN complex, has been identified. Gemin 3 interacts directly with SMN, as well as with SmB, SmD2 and SmD3. Immunolocalization studies using mAbs to Gemin 3 show that it colocalizes with SMN in gems. Gemin 3 binds SMN via its unique COOH-terminal domain, and SMN mutations found in some SMA patients strongly reduce this interaction. The presence of a DEAD box motif in Gemin 3 suggests that it may provide the catalytic activity that plays a critical role in the function of the SMN complex on RNPs.