The melanocyte-stimulating hormones (collectively referred to as MSH or intermedins) are a class of peptide hormones that in nature are produced by cells in the intermediate lobe of the pituitary gland. They were first isolated by the Yale professor Aaron B. Lerner. Synthetic analogs of these naturally occurring hormones have also been developed and researched. An increase in MSH will cause a darkening in humans too. Melanocyte-stimulating hormone increases in humans during pregnancy. This, along with increased estrogens, causes increased pigmentation in pregnant women. In Cushing's disease high levels of adrenocorticotropic hormone (ACTH) production also leads to high MSH levels, which cause an abnormal darkening.Different levels of MSH are not the major cause of racial variation in skin colour. In many red headed people, and other people who do not tan well, there are variations in their hormone receptors, causing them to not respond to MSH in the blood.