Many BB are metabolized by the liver through the CYP pathway, in particular via the CYP2D6 isoform; in humans, the CYP2D6 enzyme is highly polymorphic and this characteristic has been often referred to explain the inter-individual variability observed in the plasma levels of drugs such as carvedilol and in the responses to treatment with BB in HF. Here, CYP2D6 is linked to hydrops fetalis.