Genetic load, the concept that lies at the foundation of our constraint metric, was initially put forward by J Haldane [31] and HJ Muller [32], primarily in the context of the debate on hard versus soft selection. Here, however, we use this metric outside of such context and fully acknowledge that this is a crude, albeit computable parameter. We do not imply that a high TFBS load weakens the fitness of the individual bearing it, as would be the case in the 'classic' application of this concept. Rather, we take advantage of this concept to inquire why this probably does not occur - that is, why mutations at TFBSs are tolerated differently in different genomic contexts, likely without causing a significant reduction of an individual's fitness.
