<prompt> Why fetal cells continue to divide but terminally differentiated adult cells do not divide -There are many cyclin inhibitors which prevent cell to enter into S phase in adultPhosphatase absent in fetal cellsProteinase is absent in fetusAbsence of CD kinase  <response>: A cyclin-dependent kinase inhibitor protein is a protein which inhibits cyclin-dependent kinase. Several function as tumor suppressor genes. Cell cycle progression is negatively controlled by cyclin-dependent kinases inhibitors called CDIs, CKIs or CDKIs. CDIs are involved in cell cycle arrest at the G1 phase. 