For years, researchers have concentrated on the role of p53 in genomic stability, but this tumour suppressor has other important functions. For example, the p53-p21 checkpoint can be activated and cells arrested in G1 phase of the cell cycle by perturbation of phospholipid homeostasis. Now, Zhongmin Ma and co-workers reveal more about how this occurs (see p. 4134). Blocking the group VIA Ca2+-independent-phospholipase A2 (iPLA2), a protein that helps to regulate rapid phospholipid turnover during G1, induces cell-cycle arrest. The authors show that inhibition of iPLA2 induces the phosphorylation of p53 at Ser15 by the DNA checkpoint kinase ATR in the absence of DNA damage. In addition, they report that iPLA2 inhibition increases the ratio of polyunsaturated-to saturated-fatty-acid-containing phosphatidylcholines (PCs) in cellular membranes. The authors propose that iPLA2 normally regulates cell membrane fluidity and function by controlling the ratio of polyunsaturated to saturated fatty acids in PCs and suggest that any increase in this ratio activates ATR to postpone cell division until the problem is rectified.