CD274 and peritoneal mesothelioma: Continuing on this topic, it is not even clear whether the peritoneal mesothelioma should be distinguished from the pleural form: the former is rarer than the latter, but it seems to express higher PD-L1 levels so has not been established if a different threshold for PD-L1 expression should be used and the very small number of cases makes this assessment difficult (30, 31).