Department of Mathematics, Stockholm University, Stockholm, Sweden.|Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, United States of America.|Department of Microbiology, Tumor and Cell Biology, Karolinska Institute, Stockholm, Sweden.|Department of Mathematics, Stockholm University, Stockholm, Sweden.|Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, United States of America.