  Magombedze and Mulder in 2013 studied the gene regulatory system of
Mycobacterium Tuberculosis (Mtb) by partitioning this into three subsystems
based on putative gene function and role in dormancy/latency development. Each
subsystem, in the form of S-system, is represented by an embedded chemical
reaction network (CRN), defined by a species subset and a reaction subset
induced by the set of digraph vertices of the subsystem. For the embedded
networks of S-system, we showed interesting structural properties and proved
that all S-system CRNs (with at least two species) are discordant. Analyzing
the subsystems as subnetworks, where arcs between vertices belonging to
different subsystems are retained, we formed a digraph homomorphism from the
corresponding subnetworks to the embedded networks. Lastly, we explored the
modularity concept of CRN in the context of digraph.
