The Xerces blue (Glaucopsyche xerces) is an extinct species of butterfly in the gossamer-winged butterfly family, Lycaenidae. The species lived in coastal sand dunes of the Sunset District of San Francisco. The Xerces blue is believed to be the first American butterfly species to become extinct as a result of loss of habitat caused by urban development. The last Xerces blue was seen in 1941 or 1943 on land that is part of Golden Gate National Recreation Area. Xerces is also a group of XML software packages named after this butterfly.