Other genes previously reported to play relevant roles in this tissue showed comparable medium-low level of expression: Hdca4, a promoter of retinal cells survival (Chen and Cepko, 2009); Topors, a causative gene of retinitis pigmentosa in humans (Chakarova et al., 2007), and Pias3, encoding the SUMO E3 ligase that regulates the repressor function of NR2E3 in mouse cone genes (Onishi et al., 2009). Here, PIAS3 is linked to retinitis pigmentosa.