The apolipoprotein B gene is subject to mutations that may be important in coronary heart diseases. We have used polymerase chain reaction and denaturing gradient gel electrophoresis to characterize a single nucleotide substitution in the apolipoprotein B gene. This mutation affects amino acid 4311 of the protein and converts asparagine to serine. It was found in 24% of the 81 unrelated individuals analyzed. Moreover, another mutation was detected by sequencing in a single individual.