We used the following primary antibodies: rabbit polyclonal antibodies against SANS (RpAb) (29 (link),37 (link)), anti-SANS (mouse, monoclonal antibody; MmAb) (sc-514418, Santa Cruz), anti-FLAG (F1804, Sigma Aldrich), anti-PRPF6 (sc-48786, Santa Cruz), anti-HA (Roche), anti-GFP (ab6556, Abcam), anti-PRPF31 (PAB7154, Abnova), anti-hSNU114 (38 (link)), anti-Coilin (sc-55594, Santa Cruz), anti-SC35 (sc-53518, Santa Cruz), anti-SON (ATLAS, HPA023535), anti-SF3B1 (MmAb) (sc-514655, Santa Cruz), anti-SF3B1 (RmAb) (Will et al. 2001), anti-U5 52K (39 (link)), anti-SART1 (PA556663, Thermo Fisher) and anti-PRPF38 (40 (link)). Secondary antibodies were conjugated to the following fluorophores: Alexa488 (donkey-anti-rabbit, A21206, Molecular Probes,), CF 488 (donkey-anti-guinea pig, 20169-1, Biotrend,), Alexa488 (donkey-anti-mouse, A21202, Molecular Probes), Alexa 555 (donkey-anti-mouse, A31570, Molecular Probes), Alexa 568 (donkey-anti-rabbit, A10043, Invitrogen,) CF640 (donkey-anti-goat, 20179-1, Biotrend), Alexa 680 (donkey-anti-goat, A21084, Molecular Probes), Alexa 680 (goat-anti-rat, A21096, Molecular Probes) Alexa 680 (donkey-anti-rabbit, A10043), IRDye 800 (donkey-anti-mouse, 610–732-124, Rockland), Abberior STAR Orange (goat-anti-mouse, Abberior,), and Abberior STAR Red (goat-anti-Rabbit). Nuclear DNA was stained with DAPI (4’,6-diamidino-2-phenylindole) (1 mg/ml) (Sigma-Aldrich).