  A prominent feature of gene transcription regulatory networks is the presence
in large numbers of motifs, i.e, patterns of interconnection, in the networks.
One such motif is the feed forward loop (FFL) consisting of three genes X, Y
and Z. The protein product of x of X controls the synthesis of protein product
y of Y. Proteins x and y jointly regulate the synthesis of z proteins from the
gene Z. The FFLs, depending on the nature of the regulating interactions, can
be of eight different types which can again be classified into two categories:
coherent and incoherent. In this paper, we study the noise characteristics of
FFLs using the Langevin formalism and the Monte Carlo simulation technique
based on the Gillespie algorithm. We calculate the variances around the mean
protein levels in the steady states of the FFLs and find that, in the case of
coherent FFLs, the most abundant FFL, namely, the Type-1 coherent FFL, is the
least noisy. This is however not so in the case of incoherent FFLs. The results
suggest possible relationships between noise, functionality and abundance.
