Regulation of gene expression at the transcriptional level is achieved by complex interactions of transcription factors operating at their target genes. Dissecting the specific combination of factors that bind each target is a significant challenge. Here, we describe in detail the Allele Binding Cooperativity test, which uses variation in transcription factor binding among individuals to discover combinations of factors and their targets. We developed the ALPHABIT (a large-scale process to hunt for allele binding interacting transcription factors) pipeline, which includes statistical analysis of binding sites followed by experimental validation, and demonstrate that this method predicts transcription factors that associate with NFκB. Our method successfully identifies factors that have been known to work with NFκB (E2A, STAT1, IRF2), but whose global coassociation and sites of cooperative action were not known. In addition, we identify a unique coassociation (EBF1) that had not been reported previously. We present a general approach for discovering combinatorial models of regulation and advance our understanding of the genetic basis of variation in transcription factor binding.functional genomics | systems biology O ver the last several decades, it has become increasingly clear that the control of gene expression is due to the complex interactions of different transcription factors (TFs) working together to regulate RNA polymerase activity at promoters (1, 2). Although one factor may be a global regulator of a single process, it may function with other TFs to achieve precise regulation at different loci. Identifying the factors that work together to regulate each gene is a fundamental problem in biology.A variety of approaches have been used to measure TF coassociation. In vitro and in vivo biochemical assays have been used to detect protein-protein interactions, but these are plagued by trade-offs between sensitivity and specificity (3, 4). Moreover, in vitro assays do not always reflect the events that occur in vivo. Computational approaches have been successful in predicting binding sites by analyzing motifs in promoter regions in the context of expression data, but these are noisy and do not consider condition-specific binding events (5). Coassociation of TF binding sites can also be used to predict factors that work together (6, 7), but such approaches lack functional information about specific interactions and typically require large numbers of coassociated sites.We have recently suggested a unique approach, the Allele Binding Cooperativity (ABC) test, which uses binding variation among individuals to identify TF coassociation (8, 9). We hypothesized that variation in TF binding can occur because of sequence variation in associated TF binding sites and motifs. By searching for motifs in the binding regions for a factor of interest, the covariance of the associated motif can be correlated with binding of the factor across individuals. We demonstrated this phenomenon in a preliminary proof-of-concept, but di...