the rb1 gene inhibits the viability of retinoblastoma cells by regulating homologous recombination retinoblastoma is a childhood ocular tumor caused by the inactivation of both alleles of the retinoblastoma gene rb1 without rb1 gene function chromosomal aberrations are observed in retinoblastoma cells the instability of the genome is closely associated with the repair of dna doublestrand breaks dsbs however the precise molecular mechanism of action of rb1 in dna dsb repair remains unclear thus in this study we aimed to investigate whether the rb1 gene affects dna stability by assaying dna dsb repair and also whether it regulates the proliferation of retinoblastoma cells rb1 immunofluorescence and rtpcr were performed demonstrating that the rb1 gene is silenced in sorb50 retinoblastoma cells and the karyotype analysis of sorb50 cells indicated that the loss of rb1 function led to genomic instability both numerical and structural chromosomal aberrations were observed in our study in addition the dna dsb repair efficiency of the sorb50 cells was measured by h2ax immunofluorescence a commonly used in situ marker of dna dsbs following exposure to ionizing radiation ir 25 and 50 gy we found that the dna repair efficiency was significantly increased following irinduced damage p005 the assay for the screening of the effect of rb1 on the subpathway of dna dsb repair nonhomologous end joining nhej and homologous recombination hr indicated that rb1 did not affect nhej activity although it significantly promoted the hr pathway hr levels increased by 246fold compared with the control p001 furthermore we found that the cell viability of the sorb50 cells transfected with exogenous rb1 was significantly inhibited p001 and cell cycle assay indicated that exogenous rb1 induced s phase arrest p0001 which also inhibited the proliferation of retinoblastoma cells sorb50 in vitro therefore this study provides new insight into the mechanisms of action of the rb1 gene in regulating the proliferation of retinoblastoma cells