FMO5 and hydrops fetalis: It is well known that the main source of ROS in HF is the excessive activation of the renin-angiotensin-aldosterone system [5,54], which stimulates the NFkB signaling pathway and thus boosts production of proinflammatory cytokines, but also the activity of NADPH oxidase (NOX), which is the primary source of free radicals in the cell [8].