SMN-dependent snRNP assembly activity in PBMCs. A) 12.5 or 25 mg of extract from PBMCs or 25 mg extract from HeLa cells were incubated with biotinylated U4 snRNA or a variant of U4 lacking the Sm site (U4ΔSm) and assayed for Sm core assembly. Assembled Sm cores were isolated by immunoprecipitation with anti-Sm antibodies and detected via luminescence with HRP-coupled streptavidin. The error bars represent the standard deviation from the mean of three independent experiments. B) Representative Western blot for SMN protein in PBMC and HeLa extracts used in snRNP assembly experiments.