PROTEIN_CODING genes are represented by at least four components: TU, MODEL, EXON, CDS. The TU represents the transcriptional unit and is the highest order component of the gene.

A TU can encode multiple gene MODELs only in cases where alternative splicing exists.

A gene MODEL encapsulates all of the coding and non-coding structures of an individual splicing isoform.

Each gene MODEL can encode several mRNA EXONS and represent the spliced, intronless portions of the gene.

An mRNA EXON may only partially code for a protein; exactly the case where upstream or downstream untranslated regions exist. The protein coding portion of an individual EXON is represented by the CDS element. The CDS element will also encode the stop codon. The gene components are not ordered based on their coordinates.

For regions in which untranslated regions exist, UTR(s) will present. UTR(s) represent the non-protein-coding portions of the RNA EXON(s). UTRs are not currently supported TIGR data types outside of this DTD and they exist here only to facilitate external data analysis.