cDNA clones encoding human apolipoprotein E were identified by screening an adult human liver cDNA library with an oligonucleotide probe. The probe was a mixture of synthetic 14-base long DNA oligomers constructed to correspond to all possible codons for apo-E amino acids 218-222. Plasmids from four of the 20 clones selected by this screening procedure were digested with PstI and all had five internal PstI sites with a total length of the cDNA insert of approximately 900 base pairs. DNA sequence analysis of one of these clones, designated pE-301, revealed that it corresponded to apo-E amino acids 81-299, and contained a standard termination codon, polyadenylation signal, and poly A tail. The DNA sequence examined included the known apo-E polymorphic sites at amino acids 112, 145, and 158, and the mutant apo-E phenotypes can all be explained on the basis of a single base substitution in the first position of each of these codons. This work supports the hypothesis that the apo-E polymorphism is due to mutations in the region of DNA coding for the apo-E structural gene.