Multifactorial diseases refer to diseases having a genesis known to be influenced both by the genome and the environment. Examples include cardiovascular diseases, asthma, autoimmune diseases and various neurological and psychiatric diseases. Technical development has made possible a considerably more accurate measurement of genetic variation, having already led to the identification of hundreds of predisposing genes. We do not, however, still understand enough about the mechanisms of action of genes. The increasing progress in DNA sequencing techniques is currently changing the prospects of research more quickly than ever before.