Further improvements in the method of testing for evolutionary homology among proteins.

An earlier method for detecting significant genetic relatedness between two gene products (Fitch, 1966a) is improved upon through three specific measures. The first measure reduces the background noise, against which the homologous comparisons must be detected by eliminating large numbers of comparisons which may reasonably be expected to be non-homologous. The second measure permits the multiple simultaneous comparison of one amino acid sequence against two other known homologous sequences. The third measure modifies the log likelihood-ratio test to permit one to estimate the probability that a presumptive homologous relationship could be a chance event. The third measure is used to show that the use of the first two measures may, in some circumstances, decrease by two orders of magnitude the probability that a result would be ascribed to a chance event. The third measure is also used to show how quickly the probability that a result would be ascribed to a chance event decreases as the length of the sequences being compared is increased.