Hapalosin (80, Figure 17) is a cyclic depsipeptide, isolated from the lipophilic fraction of the extract of a cyanobacterium Hapalosiphon welwitschii W. & G.S West, and was found to reverse MDR in a P-gp overexpressing, vinblastine-resistant human ovarian adenocarcinoma cell line with higher effect than the known P-gp inhibitor verapamil [178,179]. The gene discussed is PGP; the disease is ovarian adenocarcinoma.