This e-learning module has been initiated by the European Learning Laboratory for the Life Sciences (ELLS) at EMBL. The work was done by Eoghan Harrington, Meikel Diepholz and Lukas Neidhart, who are members of the EMBL International PhD Programme, and who are supported by "E-STAR" fellowships funded by the EC's FP6 Marie Curie Host fellowship for Early Stage Research Training under contract number MEST-CT-2004-504640.