Members of the Abdominal-B-related Hox gene subfamily (belonging to homology groups 9 to 13) are coordinately expressed during limb bud development. Only two genes from homology group 13 (Hoxa-13 and Hoxd-13) are specifically expressed in the developing distal region (the autopod), which displays the most complex and evolutionarily flexible pattern among limb 'segments'. We report here that targeted disruption of the Hoxa-13 gene leads to a specific forelimb and hindlimb autopodal phenotype, distinct from that of the Hoxd-13 paralogous gene inactivation. In both limbs, Hoxa-13 loss of function results in the lack of formation of the most anterior digit and to altered morphogenesis of some 'preaxial' carpal/tarsal elements. We have generated mice with all possible combinations of disrupted Hoxa-13 and/or Hoxd-13 alleles, which allowed us to investigate the degree of functional specificity versus redundancy of the corresponding gene products in the developing limb autopod. The phenotype of any double mutant was much more severe than the sum of the phenotypes seen in the corresponding single mutants, indicating that these genes act in a partially redundant manner. Our major findings were: (1) an abnormal autopodal phenotype in Hoxa-13+/-/Hoxd-13+/- double heterozygous mutants, which mostly consists of subsets of the alterations seen in each individual homozygous mutant, and therefore appears to result from quantitative, rather than qualitative, homeoprotein deficiency; (2) partly distinct alterations in mutants harboring a single non-disrupted allele of Hoxa-13 or Hoxd-13, indicating that the remaining reduced protein amounts are not functionally equivalent; (3) a polydactyly in the forelimbs of Hoxa-13+/-/Hoxd-13-/-double mutants, consisting of seven symmetrically arranged, truncated and mostly non-segmented digits; (4) an almost complete lack of chondrified condensations in the autopods of double homozygous mutants, showing that the activity of group 13 Hox gene products is essential for autopodal patterning in tetrapod limbs.