In RB, these stem cell characteristics include the presence of a side population [19], (ie. cells that are able to exclude Hoechst 33342 dye based on expression of the drug transporter/stem cell marker ABCG2), and expression of other stem cell markers such as ALDH1, MCM2, and SCA-1 [18]. Here, ABCG2 is linked to retinoblastoma.