To this aim, we selected, from our VHIO collection of molecularly characterized breast cancer PDXs, a subset of 14 drug-PDX pairs, with 8 tumors predicted to respond and 6 predicted not to respond, namely alpelisib, an isoform-selective PI3Kα inhibitor (BYL719, n = 5); ribociclib, a CDK4/6 inhibitor (LEE011, n = 2); the combination of both (alpelisib + ribociclib, n = 3); the MEK inhibitor binimetinib (n = 2); and paclitaxel, a taxane (n = 2). This evidence concerns the gene CDK4 and breast cancer.