Among patients with familial hypercholesterolemia, half of the mutant alleles at the low density lipoprotein (LDL) receptor locus produce no immunologically detectable protein. To determine the molecular basis for one such null allele, we have cloned an abnormally short restriction fragment from the genomic DNA of one patient. The DNA sequence revealed a 5-kilobase deletion that joins a coding sequence in exon 13 to an Alu repetitive element in intron 15. The deletion joint is flanked by two inverted repeats that could potentially form a double stem-loop structure that might have predisposed to this deletion. A similar double stem-loop structure can be drawn for a previously described deletion in the LDL receptor gene and for a deletion in the beta-globin gene cluster. We speculate that such double stem-loop structures might contribute to the formation of large deletions in the human genome.