Fig. 1.

Map of the human LDL receptor gene. The gene is shown in the 5′ to 3′ orientation at the top of the diagram and is drawn to scale. Exons are denoted by filled-in areas, and introns by open areas. The regions encompassed by genomic DNA inserts in the seven bacteriophage λ and two cosmid clones are indicated at the bottom. Cleavage sites for 13 selected restriction endonucleases are shown. Asterisks denote sites that are present in the cDNA. The encircled Pvu II site is polymorphic in human populations (30). The diagonal line between exons 1 and 2 represents a gap of unknown size not present in any of the genomic clones. Additional cleavage sites for the restriction enzymes shown may be present in this gap and in intron 6 (Table 1, legend). The λ clones were isolated from 1.2 × 10⁷ plaques of a human genomic bacteriophage λ library (31). Cos1 was isolated from 6 × 10⁶ colonies of a human cosmid library (32). Cos26 was isolated from 0.9 × 10⁶ colonies of a human cosmid library (33). The libraries were screened with ³²P-labeled probes derived from the human LDL receptor cDNA, pLDLR-2 (4). Probes were isotopically labeled by nick translation (34) or hexanucleotide priming (35) and screening was carried out with standard procedures (34). Positive clones were plaque-purified or isolated as single colonies. Thirty fragments from the nine genomic clones were subcloned into pBR322 and characterized by restriction endonuclease digestion, Southern blotting (34), and DNA sequencing of exon-intron junctions (see Table 1). The restriction map was verified by comparing overlapping and independently isolated genomic clones and by Southern blotting analysis of genomic DNA isolated from normal individuals.