"The biological system in question might be a multi-step process or a
physical complex comprising several components. A common basis for
system reconstruction is orthology, where all of the components of an
experimentally characterized complex exist in one organism; if a second
organism contains orthologs of all components, then the existence of the
complex is inferred. In the case of a genome-wide analysis, a gene
product is said to fulfill a necessary function for a pathway or complex
when, with all of the other necessary functions or components also
present, the pathway or complex is reconstructed."