Abstract
We proposed a simple formula to assess the statistical significance of homologous segments found in comparison of two nucleic acid sequences (Goad and Kanehisa, Nucleic Acids Res. 10, 247-263, 1982). This paper clarifies the basic assumptions of the formula and its reliability is examined by Monte Carlo calculations. The results were satisfactory for random sequences. The formula is a useful measure for screening potentially interesting homologies and it can be implemented in any search algorithms. Examples are given for the screening procedure in the graphic display version of the Goad-Kanehisa algorithm.