In HR+ advanced breast cancer, upregulated exosomal mRNAs encoding cell cycle-regulated thymidine kinase 1 (TK1) (p = 0.01) and cyclin-dependent kinase 9 (CDK9) (p = 0.03) correlated with poor clinical response to the CDK4/CDK6 inhibitor palbociclib [173]. Here, CDK4 is linked to breast cancer.