Traditionally, the classic reductionist approach attributes functions to individual genes. For instance, this has involved the analysis of motifs or the amino acid sequences of single gene products. It is unclear how the products of particular collections genes act together to provide higher order functionality in health and disease. To address this higher order problem, the function of collections of genes, as opposed to "one gene at a time" has to be studied. Accordingly, a model system is needed to test systems biology. In our studies, we used the homocysteine-folate metabolism as a model system.