 T92, the Tamura 1992 model, is a mathematical method developed to estimate the number of nucleotide substitutions per site between two DNA sequences, by extending Kimura's (1980) two-parameter method to the case where a G+C content bias exists.This method will be useful when there are strong transition-transversion and G+C-content biases, as in the case of "Drosophila" mitochondrial DNA.T92 involves a single, compound base frequency parameter formula_129 (also noted formula_130) formula_131

As T92 echoes the Chargaff's second parity rule — pairing nucleotides do have the same frequency on a single DNA strand, G and C on the one hand, and A and T on the other hand — it follows that the four base frequences can be expressed as a function of formula_130

formula_133 and formula_134

Rate matrix formula_135

The evolutionary distance between two DNA sequences according to this model is given by
where formula_137 and formula_138 is the G+C content (formula_139).