Dacic S, Roy S, Lyons MA, et al. - Researchers conducted whole exome sequencing (WES) to determine the genetic events that result in the development of mesothelioma in situ. They also investigated if somatic or germline mutations are present in cases of mesothelioma in situ. This analysis involved two cases of mesothelioma in situ (1 pleural, 1 peritoneal). Copy number loss and LOH in the BAP1 locus on chromosome 3 were found in the case of pleural mesothelioma in situ. Both a BAP1 somatic splice site mutation involving intron 5-exon 6 boundary (S126_splice) with an allelic fraction of 10%, and BAP1 copy number loss were detected in the peritoneal mesothelioma in situ. Overall, findings of WES corroborated an association of BAP1 somatic mutations/deletions with the development of mesothelioma in situ, as well as indicated that BAP1 mutation/deletion is a very early event in malignant mesothelioma development. It is yet to be ascertained if BAP1 mutation/deletion alone is enough to cause invasive mesothelioma or if additional genetic changes are needed.