Network-level conservation filter. (a) The occurrence of a candidate TFBS in the set of orthologous Arabidopsis-poplar gene pairs was determined and the significance of the overlap is measured using the hypergeometric distribution . The NCS is defined as the negative logarithm of the hypergeometric p value. (b) Distribution of NCS values for 1,000 randomly generated TFBSs (grey) and the motifs found using the co-expression (black) and the two-way clustering (white) procedure. The left and right y-axis show the frequency for the random and the potentially functional TFBSs, respectively.