We additionally verified the expression of the identified marker sets in four RB tumor cell lines (Y79, WERI-Rb1, RB355 and Rbl-13), showing that all identified RB tumorsphere markers, except for GNGT2, CRB1 and RAX2, were also expressed in the RB tumor cell lines, whereas the stromal markers were not expressed (Fig. 3c, d; delta Ct higher than 10 equates to Ct value > 30, defined as not expressed). This evidence concerns the gene RB1 and retinoblastoma.