HAIR-AN syndrome consists of hyperandrogenism (HA), insulin resistance (IR), and acanthosis nigricans (AN).
Although it is a rare subset of polycystic ovary syndrome, studies on patients with HAIR-AN played a key role in elucidating the pathogenesis of the former. In particular, although HAIR-AN results from two very different types of abnormalities - blocking antibodies against the insulin receptor OR genetically absent/reduced insulin receptor number/function - patients with both types have high levels of androgens (male hormones).
Research on the two types of HAIR-AN demonstrated that patients with both forms of HAIR-AN had very high levels of insulin and, critically, that it was the high insulin that caused the elevation in androgens. In other words, regardless of how it comes about, high levels of insulin lead to high levels of androgens. This insight had important implications for the understanding of the more common form of polycystic ovarian syndrome and ultimately led to new treatments for it, such as metformin.