Field inversion gel electrophoresis (FIGE) of SfiI-digested chromosomal DNA was used to demonstrate a 50-kb deletion in one allele of the gene encoding the beta subunit of human hexosaminidase (HEXB at 5q13) of two apparently unrelated patients with Sandhoff disease. In conventional electrophoretic restriction analysis, this deletion was masked by hybridization of bands from the other allele.