A nested intronic gene lies within the non-coding intronic region of a larger gene, and occurs relatively frequently, especially in the introns of metazoans and higher eukaryotes. Because only eukaryotic DNA contains intronic regions, this type of gene does not occur in bacteria or archaea.
The human genome contains a relatively high proportion of nested intronic genes. It is predicted to contain at least 158 functional intronic nested genes, with an additional 212 pseudogenes and three snoRNA genes nested in intronic regions. These genes seem to be distributed randomly across all chromosomes, and the majority code for proteins that are functionally unrelated to their host genes.