A group of rare Mendelian diseases caused by pathogenic variants in genes encoding core spliceosomal complexes have been characterized as spliceosomopathies, e.g., SF3B4 involved in the U2 snRNP in Nager syndrome, an acrofacial dysostosis; SNRPB of the U1/2/5/4/6 snRNP in cerebrocostomandibular syndrome, characterized by Robin sequence and rib defects; and U4/U6 snRNP subunits (PRPF3, PRPF4, and PRPF31) and U5 snRNP subunits (PRPF6, PRPF8, and SNRNP200) in retinitis pigmentosa. The gene discussed is PRPF4; the disease is retinitis pigmentosa.