Division of Bioinformatics, Department of Epidemiology and Biostatistics, University of California, 550 16th Street, San Francisco, 94158, CA, USA. mark@biostat.ucsf.edu.|Division of Bioinformatics, Department of Epidemiology and Biostatistics, University of California, 550 16th Street, San Francisco, 94158, CA, USA. henrik.bengtsson@ucsf.edu.