Adrenocorticotrophin (ACTH) insensitivity is a potentially lethal inherited disorder of ACTH signalling in the adrenal. Inactivating mutations of the ACTH receptor account for ∼25% of these cases. A second genetic cause for this syndrome has recently been identified in the MRAP gene. The MRAP protein appears to function in the trafficking and cell surface expression of the ACTH receptor, and might indicate the existence of more widespread G-protein-coupled receptor trafficking mechanisms. Molecular defects underlying other causes of ACTH insensitivity syndromes will probably contribute further to our understanding of these pathways.