In what follows, we demonstrate (i) that the previously reported excess of nonsense mutations in RB1 at CpGs is compatible with the elevated rate of mutation at those sites, refuting a specific pathogenic mechanism in RB, (ii) an enrichment of essential splice-site donor mutations at exon 6 and 12, but depletion at exon 5, indicative of previously unappreciated heterogeneity in relative penetrance across this type of putative LoF mutation, and (iii) a statistically significant excess of mutations found at Arg661Trp in bilateral RB, as a hotspot for missense mutations with lower penetrance. Here, RB1 is linked to retinoblastoma.