FNDC5 and hydrops fetalis: Furthermore, we show that levels of Ucp1 which would act to metabolise fat are dramatically decreased in the adipose tissue of Mstn−/− mice in response to high fat possibly due to a decrease in the expression of FNdc5/irisin. Since the irisin-Ucp1 pathway that regulates the development of brown adipose tissue (BAT) was perturbed in Mstn−/− HF mice, we measured transcript levels of key genes that promote BAT (i.e. Cidea, PR domain zinc finger protein 16; Prdm16; proton assistant amino acid transporter-2, Pat2; Fig. 9a).