Like all vertebrates, humans establish anatomical left-right asymmetry during embryogenesis. Variation from this normal arrangement (situs solitus) results in heterotaxy, expressed either as randomization (situs ambiguus) or complete reversal (situs inversus) of normal organ position. Familial heterotaxy occurs with autosomal dominant, recessive and X-linked inheritance. All possible situs variants, solitus, ambiguus and inversus, can appear among some heterotaxy families. Positional cloning has led to the identification of a gene on the X chromosome responsible for some cases of human heterotaxy. Additional candidate genes have emerged from recent studies of left-right axis development in chick, frog and mouse, which have begun to elucidate a tightly regulated genetic cascade that differentiates the left and right sides prior to the appearance of morphological asymmetry.