The histone-modifying enzyme lysine-specific histone demethylase 1A (LSD1) is overexpressed in many human tumours, and its inhibition reduces cancer cell growth, migration and invasion. Mohammad et al. report the discovery and biochemical characterization of an orally available, cyclopropylamine-containing, irreversible LSD1 inactivator, GSK2879552. The small molecule inhibited proliferation of small cell lung cancer (SCLC) cell lines and delayed tumour growth in SCLC xenograft-bearing mice. A DNA hypomethylation signature was found to be predictive of the sensitivity of patient-derived xenograft models of SCLC to the antiproliferative effects of GSK2879552.