 Though the Jukes-Cantor model is still used, more sophisticated models of evolution are available, e.g., the K80 (Kimura 1980), HKY85 (Hasegawa, Kishino, &amp; Yano 1985) and Tamura-Nei (Tamura &amp; Nei 1993) models of evolution. These introduce more parameters, e.g., assumptions about equal frequencies of nucleotides and/or the uniform probability of changing nucleotide states may be relaxed and even customized; models may also incorporate molecular clock assumptions.