The gene coding for sucrase-isomaltase (SI) has recently been mapped to chromosome 3 using a cDNA probe to analyse DNA from somatic cell hybrids (Green et al. 1987). We have now used this same cDNA probe to obtain a regional localization of this gene. In situ hybridization to normal metaphase chromosomes and to chromosomes from individuals with balanced translocations suggests a regional assignment to chromosome 3q25-26.