RB1 and retinoblastoma: Subtype 1 retinoblastomas (equivalent to cluster 2 in Kooi et al.13) are characterized by few genetic and chromosomal alterations, mostly limited to RB1 loss and 6p gain, and show gene expression resembling signatures of maturing photoreceptor cells that Kooi et al. termed a “photoreceptorness signature”13 and Liu et al. termed a “cone marker signature”12.