sentences:  Fos/Jun/Myc helps to activate the Cyclin D/Cdk4 complex.Phosphatidylinositol 3 kinase phosphorylates p27 (or p16 or p21) and SCF/Skp1.The phosphorylation of p27 inhibits p27's ability to bind Cdk4, thus freeing Cdk4 to associate with Cyclin D and form an active complex.