Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons. Is recruited by release factors to stalled ribosomes together with SMG1 and SMG8 (forming the SMG1C protein kinase complex) and, in the SMG1C complex, is required for the efficient association between SMG1 and SMG8 (By similarity). Plays a role in brain, heart, and eye development. Self-associates to form homodimers and forms heterodimers with SMG8; these assembly forms may represent SMG1C intermediate forms (By similarity). Component of the SMG1C complex composed of SMG1, SMG8 and SMG9 (By similarity). Interacts with DHX34; the interaction is RNA-independent (By similarity). Phosphorylated by SMG1. Belongs to the SMG9 family.