Biocomputing Unit, National Center of Biotechnology (CSIC), Department of Computer Science, University Autonoma de Madrid, Campus Universidad Autonoma s/n, 28049 Cantoblanco, Madrid, Spain, Department Applied Mathematics, Tel Aviv University, Ramat Aviv, Tel Aviv 69978 Israel and Bioengineering Lab, Escuela Politecnica Superior, University San Pablo CEU, 28668 Boadilla del Monte, Madrid, Spain.