The synthetic oligonucleotide MF59, derived from the rabbit sucrase-isomaltase cDNA sequence, hybridizes to two classes of mRNA in the rat and pig intestinal epithelium as well as to various restriction fragments within the human genome. These results strongly support the hypothesis that the sucrase-isomaltase gene belongs to a gene family. The cloning of a human cosmid containing such a sequence similar to MF59 has allowed us to identify a new member of this gene family which detects a single 6.5 Kb intestinal mRNA in the adult pig.