RB1 and cancer: To address these limitations, we describe here a proteogenomic approach that allows the identification of RB1-defective cancers as well as cancers that phenocopy RB1-defective cancers, a class of cancers we term as “RBness.” Part of our reason for defining RBness cancers is that these are likely to behave in much the same way as RB1 mutant cancers and could be treated in a similar way.