{17beta-Hydroxysteroid} dehydrogenase type 1 {(HSD17B1)} is a key enzyme of 17beta-estradiol biosynthesis, and in rodents is additionally involved in testosterone biosynthesis. The human {HSD17B1} gene, located on chromosome 17q12-21, is duplicated in tandem, with the 3???-copy being the functional gene. Here we show by sequencing the gene from a diverse set of related species that this duplication is of very recent evolutionary origin, having occurred in the common ancestor of Hominoidae (apes and humans) while being absent in the closely related Old World monkeys {(Macaca)} and the outgroup species Tupaia belangeri and Mus musculus. By computational analysis of the conserved regulatory elements in the 5???-untranslated {(5???-UTR)} and putative promoter region of the {HSD17B1} gene and, where present, pseudogene, across our broad sample of species we can show significant differences that might point to the origin of the divergent substrate specificity of human and rodent {HSD17B1} and highlight potential functionally relevant differences in regulatory patterns in different evolutionary lineages. ?? 2005 Elsevier Ireland Ltd. All rights reserved.