PATRONUS (from "PATtern Recognition by Optimized Numerical Universal Scoring") is a program designed to compute in a very fast way the exact probability of observing a given number of occurrences of a simple motif (that is, a continuous word without gaps) in a sequence. Its intended scope is the analysis of very long biological sequences, like chromosomes or whole genomes of complex organisms. The probability is computed on the basis of the Markovian statistics of order m for the sequence, that is the recorded number of the occurrences of all the submotifs of length m + 1 in the sequence. Contrary to what many people believe, computing such a probability for a generic motif is a computationally demanding task, mainly because motifs can overlap in non-trivial ways.
A detailed description of both the PATRONUS algorithm and its excellent performance can be found here.