A new variant of the low density lipoprotein receptor (LDLR) gene was ascertained through Southern blotting analysis of LDLR genes of 35 unrelated Japanese patients with heterozygous familial hypercholesterolemia (FH). This mutant gene had a 6 kilobase deletion which had eliminated only exon 15, an exon that encodes the O-linked sugar domain. The mutation was recognized in two patients with heterozygous FH. We refer to these patients as 'FH-Tonami', since they were both born in the Japanese district of Tonami. Although there is no evidence of a relation between families, the possibility of a common ancestor with FH does exist. Neonatal diagnosis of FH in two fetuses from one family was possible through analyses of their LDLR genes in cord blood samples at delivery.