CDK4 and neoplasm: A new CDK4/ARK5 inhibitor 8-cyclopentyl-2-[4-(4-methylpiperazin-1-yl)-phenylamino]-7-oxo-7,8-dihydropyrido[2,3-d]pyrimidine-6-carbonitrile, also known as 7x, was shown to lead to a dose-dependent inhibition of tumor growth and reduction in tumor weight over a 21 day period in nude mice breast cancer model [37].