CDKN1A and regulation of cell cycle: Once mTOR is commonly phosphorylated at position 2448 via the PI3K/Akt and has been inhibited when higher levels of p15INK4B, p16 INK4A, p21Cip1, p27Kip1, p53, and other suppressor tumors are present under stress conditions, p21 obliges G1 restriction by inhibitory binding to CDK2/cyclin E or other CDK/cyclin complexes [45].