THE morphogenesis of mammalian digits requires the function of several genes of the HoxD complex during development of limb buds(1-4). Using embryonic stem (ES) cells and a site-specific recombination system (loxP/Cre), we have induced a deficiency(5,6) that eliminates the products of the Hoxd-13, Hoxd-12 and Hoxd-11 genes simultaneously. A Hoxd-11/lacz reporter gene replaced the deleted region in order to monitor the effect of this triple inactivation at the cellular level. Mice homozygous for this deficiency showed small digit primordia, a disorganized cartilage pattern and impaired skeletal mass. These alterations are similar to the defects seen in a human synpolydactyly(7,8), suggesting that this syndrome, which is associated with a subtle mutation in HOXD13 (ref. 8), may involve the loss of function of several Herd genes. These results indicate the existence of a functional hierarchy among these genes and provide us with an animal model to study human digit malformations.