MKI67 and neoplasm: At a postnatal age of 60 days (P60), when pineal cell proliferation is continuing and prior to invasive tumor development, which is seen by 3 months of age (18), p53 restoration for a period of 10 days (P60 to P70) resulted in cell cycle exit, as evidenced by the loss of Ki67 positivity (Fig. 2C, column 1, compare top and middle rows, and D).