CDKN1A and glioblastoma: Despite the number of independent factors that regulate p21, it is heartening to see that APTR and p21 levels were anti-correlated in glioblastomas, suggesting that at least in these tumors APTR is a significant repressor of p21. We do not know whether the variation in levels of APTR stem from genetic differences between the tumors or differences in the growth rate, oxygenation or other factors that affect the level of stress on the tumors.