GSTM1 and endometriosis: However, since inducible HO-1 was weakly expressed by macrophages and mesothelial cells that make up the majority of cells in the peritoneal cavity, and because there was no concomitant upturn in peritoneal fluid levels of its final by-product bilirubin, it strongly infers that detoxifying systems, while present, may be inadequate to metabolize Hb in the case of endometriosis [37,43].