RP9 (retinitis pigmentosa 9) is a pre-mRNA splicing factor that localizes to nuclear speckles and directly interacts with other splicing factors including U2AF35 1. The protein functions as a splicing regulator whose activity is dependent on its phosphorylation state 1. RP9 has been identified as an interaction partner of ADH1C, working together to enhance splicing of peroxisome proliferator-activated receptor alpha (PPARα) pre-mRNA, which regulates fatty acid degradation 2. Mutations in RP9 cause autosomal dominant retinitis pigmentosa (adRP), accounting for a subset of adRP cases characterized by early-onset and severe vision loss 34. The D170G mutation impairs splicing activity and reduces PAP-1 phosphorylation levels, potentially causing RP through altered splicing of retinal genes 1. In photoreceptor cells, RP9 mutations decrease cell proliferation and migration, and disrupt pre-mRNA splicing of retinitis pigmentosa-associated genes including Fscn2 and Bbs2 3. RP9 mutations cause retinal-specific global spliceosome dysregulation affecting genes involved in phototransduction, photoreceptor morphogenesis, and retinal cell organization 5. Why spliceosomal protein mutations manifest as tissue-specific retinal disease despite ubiquitous expression remains incompletely understood 4.