Familial defective apolipoprotein B-100 (FDB) is caused by a point mutation in exon 26 of the apolipoprotein B gene leading to a decreased binding to the LDL-receptor. Patients with FDB have hypercholesterolemia and atherosclerotic disease. Since other mutations of apoB-100 could also cause binding abnormalities we established a temperature-gradient gel electrophoresis (TGGE) method and started to screen hypercholesterolemic patients for the presence of point mutations in this region. 4 of 43 patients were positive according to TGGE and subsequent sequencing showed the familiar guanine to adenine transition in codon 3500 in all cases.