A 23-nucleotide tandem duplication (GGACCTTGAAAGTACTC-GGGACC) was found within exon 3 of the beta-galactosidase gene in a patient with infantile-form GM1-gangliosidosis, which generated a premature stop codon after translation of 36 amino acids. Homologous sequences at the area of duplication suggested that the mutation resulted from an unequal crossover. A single base substitution 316Trp----Cys was found in the other allele. Family study showed that the duplication was transmitted from his father and the base substitution from his mother.