RB1 and gastrointestinal stromal tumor: Because the whole exome sequencing results in our present series underscored the importance of genomic inactivation of cell cycle regulator genes in GIST, we performed focused testing of the genomic status (mutation, copy number variation, and LOH) of three of these genes (CDKN2A, RB1, and TP53) in an independent group of GIST patients with annotated long-term clinical follow up (validation data set).