A central challenge in systems biology today is to understand the network of interactions among biomolecules and, especially, the organizing principles underlying such networks. Recent analysis of known networks has identified small motifs that occur ubiquitously, suggesting that larger networks might be constructed in the manner of electronic circuits by assembling groups of these smaller modules. Using a unique process-based approach to analyzing such networks, we show for two cell-cycle networks that each of these networks contains a giant backbone motif spanning all the network nodes that provides the main functional response. The backbone is in fact the smallest network capable of providing the desired functionality. Furthermore, the remaining edges in the network form smaller motifs whose role is to confer stability properties rather than provide function. The process-based approach used in the above analysis has additional benefits: It is scalable, analytic (resulting in a single analyzable expression that describes the behavior), and computationally efficient (all possible minimal networks for a biological process can be identified and enumerated). Prior work on network decomposition-understanding a network's components-has focused on two types of analysis. The first, which we will call motif occurrence analysis, examines all possible small motifs with two, three, or four nodes and by searching for these motifs in known networks, identifies those motifs that occur most frequently across all known networks (7-9). The assumption is that frequently occurring motifs then form a useful building block or module that confers some functionality or property. The second type of work, which we will call motif function analysis, focuses more closely on network function or dynamics. This approach starts with a given network and its known dynamic behavior (the function of the network) and, by removing the edges in a small motif, tries to characterize the effect of the motif. The thinking here is if the removal of a motif results in a loss of function, the motif can be said to contribute to the function. Note that, because any subset of connected edges can be a plausible motif, the number of trials needed for a systematic search of all motifs grows exponentially large, a limitation that also afflicts the motif-occurrence approach. These approaches leave open the question: Do networks contain large motifs that are a primary determining factor in achieving a network's function?In this paper, we present a unique approach to decomposition that addresses the above large-motif question in the affirmative. This approach, which we call process-based analysis, starts by characterizing the space of all possible networks that provide the desired function (process) and then identifies, among these, the minimal networks (with the fewest edges). These minimal networks, it turns out, are few in number and capture the primary functionality-the removal of any single edge from a minimal network destroys the network's function. Thus, suc...