Melanocyte stimulating hormones, collectively known as MS or intermedins, are a class of peptide hormones produced by cells in the middle lobe of the pituitary gland. The increase in MSH causes the person to turn black. Melanin increases during pregnancy, along with an increase in estrogen, causing hyperpigmentation in pregnant women. Cushing's syndrome is due to hyperpigmentation due to excess adrenocorticotropic hormone (ACTH), such as axillary acanthosis nigricans. Most people with primary Addison's have darkening of the skin (hyperpigmentation), including areas that are not exposed to the sun, and are characterized by skin folds (such as in the hands), nipples, and the inside of the cheeks (buccal mucosa), and old scars may darken. This is because melanin (MSH) and adrenocorticotropic hormone (ACTH) have the same precursor molecule, promelanocortin (POMC).