Loss of expression of BRCA1 associated protein (BAP1) or methylthioadenosine phosphorylase (MTAP) or homozygous deletion of cyclin‐dependent kinase inhibitor 2A (CDKN2A) (p16) by FISH helps to distinguish reactive mesothelial cells from malignant pleural mesothelioma. The gene discussed is MTAP; the disease is malignant pleural mesothelioma.