Many client proteins of this chaperone (e.g. Raf-1, Akt, ATM, CDK4, HIF1α, ErbB2, BRCA1/2, survivin and others) are key components of signaling pathways responsible for unlimited proliferation of cancer cells, their resistance to apoptosis, repair of damaged DNA etc. Dysfunction of Hsp90 leads to inactivation and degradation of those client proteins, so that cell-permeable inhibitors of the Hsp90 activity can block multiple Hsp90-dependent reactions ensuring survival and proliferation of cancer cells [6]. Here, CDK4 is linked to cancer.