RB1 and retinoblastoma: Motivated by the studies in mice where it was shown that bi-allelic mutations in both the Rb1 and Rbl1 genes induced retinoblastoma, we wanted to investigate whether this was also the case in Xenopus. In order to ensure that rbl1 mutant animals are tumor-free, two-cell stage X. tropicalis embryos were unilaterally injected with rbl1 coding region 1 (rbl1cr1) gRNA.