CDK4 and neoplasm: A significant number of Hsp90 client proteins are oncoproteins, such as receptor tyrosine kinases; human epidermal growth factor receptor 2 (Her2), epidermal growth factor receptor (EGFR), mesenchymal epithelial transition factor receptor (Met), rapidly accelerated fibrosarcoma-C (c-Raf), cell cycle regulators cyclin dependent kinase 4 (Cdk4), and hypoxia-inducible factor 1α (HIF-1α), which are necessary for the malignant transformation and tumor development [4,5].