A defective LDL receptor gene in a child with familial hypercholesterolemia produces a receptor precursor that is 50,000 daltons larger than normal (apparent Mr 170,000 vs. 120,000). The elongated protein resulted from a 14 kilobase duplication that encompasses exons 2 through 8. The duplication arose from an unequal crossing-over between homologous repetitive elements (Alu sequences) in intron 1 and intron 8. The mutant receptor has 18 contiguous cysteine-rich repeat sequences instead of the normal nine. Seven of these duplicated repeats are derived from the ligand-binding domain, and two repeats are part of the epidermal growth factor precursor homology region. The elongated receptor undergoes normal carbohydrate processing, its apparent molecular weight increases to 210,000, and the receptor reaches the cell surface where it binds reduced amounts of LDL but undergoes efficient internalization and recycling. The current findings support an evolutionary model in which homologous recombination between repetitive elements in introns leads to exon duplication during evolution of proteins.