In this study we report the first single cell RNA-and ATAC-Seq analyses of two retinoblastoma tumors, caused by biallelic mutations in the RB1 gene, revealing the dominant presence of proliferating cone precursors at different stages of cell cycle in the tumors and identifying G2/M cone precursors as the cell of origin for Rb. Here, RB1 is linked to retinoblastoma.