  Background: Qualitative frameworks, especially those based on the logical
discrete formalism, are increasingly used to model regulatory and signalling
networks. A major advantage of these frameworks is that they do not require
precise quantitative data, and that they are well-suited for studies of large
networks. While numerous groups have developed specific computational tools
that provide original methods to analyse qualitative models, a standard format
to exchange qualitative models has been missing.
  Results: We present the System Biology Markup Language (SBML) Qualitative
Models Package ("qual"), an extension of the SBML Level 3 standard designed for
computer representation of qualitative models of biological networks. We
demonstrate the interoperability of models via SBML qual through the analysis
of a specific signalling network by three independent software tools.
Furthermore, the cooperative development of the SBML qual format paved the way
for the development of LogicalModel, an open-source model library, which will
facilitate the adoption of the format as well as the collaborative development
of algorithms to analyze qualitative models.
  Conclusion: SBML qual allows the exchange of qualitative models among a
number of complementary software tools. SBML qual has the potential to promote
collaborative work on the development of novel computational approaches, as
well as on the specification and the analysis of comprehensive qualitative
models of regulatory and signalling networks.
