Previous studies have hypothesized that low levels of the enzyme thymidylate synthase (TS) likely mark patients who will benefit from the drug pemetrexed – but results have been inconclusive at best and at times contradictory. A new study provides an explanation why: Only in combination with high levels of a second enzyme, FPGS, does low TS predict response to pemetrexed in patients with malignant pleural mesothelioma.