MANY homeobox genes are clustered in a linear array along a chromosome, reflecting their ordered expression along the anterior-posterior axis of the embryo(1). Expression patterns(2,3) as well as grafting(4), ectopic expression(5,6) and loss-of-function experiments(7-11) suggest that the Hox genes encode a combinatorial system of positional specification along that axis. In contrast, the function of orphan homeobox genes(12) located at sites outside the four mammalian Hox clusters is less well understood. To assess the functional role of the orphan homeobox gene Hox11, we have generated Hox11-deficient mice through gene targeting. Hox11(-/-) mice have no spleen, but otherwise appear normal. Hox11 is normally expressed in the splenic anlage arising from the splanchnic mesoderm. Hox11(-/-) embryos have no cellular organization at the site of splenic development but all other splanchnic derivatives develop normally. Hox11 controls the genesis of a single organ, providing new insight into the genetic regulation of morphogenesis.