TP53 and cancer: To better understand the complex mechanism that differently regulates iSHMTs-induced apoptosis we also studied p53, known to have a pivotal role in the control of cancer cell proliferation and survival.17 The H1299 cell line does not express p53, because of a deletion of this gene, but undergoes cell cycle arrest after iSHMT1, 2 and 1+2 transfection, thereby demonstrating that, in this model, cell cycle arrest is largely p53-independent.