REN and Hyperkalemia: Recently published data on the MC2R knockout (KO) mice revealed that these mice have low serum aldosterone levels leading to hyperkalaemia in females, and increased expression of the angiotensin receptor 1b (AT1bR) in the ZG, implying an element of compensation in the renin–angiotensin–aldosterone axis in the complete absence of MC2R.12,13 Furthermore, Lin et al. recently described a series of children with MC2R mutations who had a degree of increased renin activity or aldosterone deficiency at initial investigation and were treated with 9α-fludrocortisone.