CDKN1A and neoplasm: The S-phase reduction is also attributable to down-regulation of cyclin-dependent kinase inhibitor 1A of p21 (i.e. p21CDKN1A, Figs 12a and 13a7), which resulted from impaired expression of its upstream p53, a conserved tumour repressor controlling cell cycle division and progression (Fig. 13a6).