After these observations, the mesothelioma in situ is for the first time included in the current WHO classification, and the essential criteria for its diagnosis are: non-resolving pleural effusion, no thoracoscopic or imaging evidence of tumor, single layer of mesothelial cells on the surface without invasive growth, and loss of BAP1 and/or MTAP and/or CDKN2A homozygous deletion, and multidisciplinary discussion of the diagnosis. This evidence concerns the gene BAP1 and neoplasm.