Further definitions of the BA/FXR/FGF19/FGFR4 axis phenotypes, and their relevance to disease, whether primary, caused by genetic polymorphisms, or secondary to acquired factors such as obesity or metformin, is an area of on-going research, both in BAD[32] and in NAFLD and NASH. Here, FGF19 is linked to obesity due to melanocortin 4 receptor deficiency.