RP9 and Alzheimer disease: In 2005, Ziviello et al. [46] conducted a study on 43 Italian families to investigate the presence of variants in 12 genes (RHO, RDS, RP1, IMPDH1, PRPF31, CRX, NRL, FSCN2, HPRP3, RP9, CA4, and PRPF8) associated with AD-RP.