In the course of developing a low-density lipoprotein receptor (LDLR) gene therapy treatment for homozygous familial hypercholesterolemia (HoFH), we planned to examine the efficacy in a nonhuman primate model, the rhesus macaque heterozygous for an LDL receptor mutation fed a high-fat diet. Unexpectedly, our initial cDNA sequencing studies led to the identification of a heretofore unidentified splicing isoform of the rhesus LDLR gene. Compared with the publicly available GenBank reference sequence of rhesus LDLR, the novel isoform contains a 21 bp in frame insertion. This sequence coincides with part of exon 5 and creates a site for the restriction enzyme MscI. Using this site as a marker for the 21 bp in-frame insertion, we conducted a restriction enzyme screen to examine for the prevalence of the novel isoform in rhesus liver tissue cDNA and its homolog in human liver tissue cDNA. We found that the novel isoform is the predominant LDLR cDNA found in rhesus liver and the sole LDLR cDNA found in human liver. Finally, we compared the in vivo functionality of the novel and previously identified rhesus LDLR splicing isoforms in a mouse model of HoFH.