A method and system for determining whether a sequence fragment g is atypical with respect to a reference sequence G using compositional methods and including constructing a template from G and g respectively containing a sequence of characters for a comparison with one another, wherein a number of characters contained in the template exceeds two. For the case where the sequences at hand are genetic, the atypicality detection can be used to determine whether a given sequence fragment g is the result of a horizontal transfer event.