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The mitochondrial genome includes just 13 protein-coding genes that might serve as the core of a DNA-based identification system. The length of each gene varies little across the animal kingdom, but there is more than a 10-fold difference among genes. This length variation acts to exclude most genes from consideration, as a minimum length of 900 bp is required to make best use of existing technologies. Current sequencers may only routinely recover 700 BP, but internal PCR primers must be used to generate the DNA for sequencing to avoid complications which would arise from shifts in gene order if external primers were used. In fact, only four mitochondrial genes have a minimum length in excess of this 900 BP threshold - COI, Cyt b ND4, and ND5.