Apolipoprotein E (apoE) is associated with plasma lipoproteins and is important 
in modulating the catabolism of remnants of triglyceride-rich lipoproteins. ApoE 
is a polymorphic protein, and homozygosity for the E2 allele is associated with 
type III hyperlipoproteinemia. A variant of apoE, apoEBethesda, was found that 
migrates in the E1 position on isoelectric focusing. Evaluation of the proband 
and her family showed that both individuals with apoEBethesda had type III 
hyperlipoproteinemia, and the analysis was consistent with the concept that 
apoEBethesda was coded by a new E allele and that this allele was expressed in a 
codominant fashion. The discovery of apoEBethesda and its association with type 
III hyperlipoproteinemia provides additional evidence for the genetic 
heterogeneity of the E allele and suggests that a number of different structural 
mutations of the E apolipoprotein may be associated with the type III phenotype.