The different types of sequence model make differing assumptions about how and where motif occurrences appear in the dataset. The simplest model type is OOPS (One-Occurence-Per-Sequence) since it assumes that there is exactly one occurrence per sequence of the motif in the dataset. This is the case we have analyzed in the Gibbs sampling section. This type of model was introduced by Lawrence & Reilly (1990) , when they describe for the first time a generalization of OOPS, called ZOOPS (Zero-or-One-Occurrence-Per-Sequence), which assumes zero or one motif occurrences per dataset sequence. Finally, TCM (Two-Component Mixture) models assume that there are zero or more non-overlapping occurrences of the motif in each sequence in the dataset, as described by Baily & Elkan (1994).  Each of these types of sequence model consists of two components, which model, respectively, the motif and non-motif (background) positions in sequences. A motif is modelled by a sequence of discrete random variables whose parameters give the probabilities of each of the different letters (4 in the case of DNA, 20 in the case of proteins) occurring in each of the different positions in an occurrence of the motif. The background positions in the sequence are modelled by a single discrete random variable.