McGill Centre for Bioinformatics, McGill University, Montreal, Quebec, Canada. perkins@mcb.mcgill.ca|McGill Centre for Bioinformatics, McGill University, Montreal, Quebec, Canada. perkins@mcb.mcgill.ca|McGill Centre for Bioinformatics, McGill University, Montreal, Quebec, Canada. perkins@mcb.mcgill.ca|McGill Centre for Bioinformatics, McGill University, Montreal, Quebec, Canada. perkins@mcb.mcgill.ca