Alternative splicing of mRNA is an important mechanism for organisms to enhance protein diversity from a limited number of genes. In this report, we described a novel exon insertion in the interleukin 23 (IL-23) receptor between exons 9 and 10, denoted as exon 9a. This 162 base-pair insertion was the only insertion variant discovered in more than 20 IL23R deletion variants found in the mRNA of mitogen-stimulated peripheral blood mononuclear cells (PBMC). Sequence analysis revealed that a pair of GT/AG splice donor-acceptor site and several putative cis-acting sequences were present; the insertion was identified throughout the genome and found to contain homology to the L1 retrotransposon protein. This report describes an insertion in the IL-23 receptor and due to consequent early termination within the intracellular region, causing a possible non-responsive receptor isoform.