PURPOSE. To assess whether the Pax6 gene is involved in the development of retinoblastoma. METHODS. Three human retinoblastoma cell cultures were transfected with human Pax6 specific double-stranded, small interfering siRNA molecules RH-1 and RH-2. In addition, untreated control groups and negative control groups (CT groups) transfected with siRNA without homology to the human genome were formed for all three cell culture lines. RESULTS. After Pax6 gene was silenced by siRNA, the percentage of tumor cell survival decreased significantly (P < 0.05). Correspondingly, the percentage of apoptotic cells to total cells was significantly (P < 0.05) higher in the three retinoblastoma cell lines transfected with siRNA than in the CT control groups and the untreated control groups. In a parallel manner, the cell cycle was significantly (P < 0.01) altered in the transfected study groups, with reduced percentages of retinoblastoma cells in the S-phase. The cell-cycle-associated protein P21 was upregulated, and the protein P27 was slightly upregulated in the transfected retinoblastoma cell lines, in comparison to the control groups. CONCLUSIONS. Silencing the Pax6 gene with short interfering RNA resulted in an inhibited growth and an increased apoptosis of cultured human retinoblastoma cells. It was paralleled by upregulation of the P21 and P27 proteins. (Invest Ophthalmol Vis Sci. 2011;52:4560-4570) DOI:10.1167/iovs.10-5487