Positional cloning reveals that hob, an AV conduction block mutation, affects the homeobox transcription factor, Tcf2. Given the increased prevalence of AV block in humans with type II diabetes mellitus independent of congestive heart failure and coronary artery disease [42], Tcf2, a mature onset diabetes of the young/diabetes-associated gene, may also directly mediate the development and maintenance of mammalian AV conduction. This evidence concerns the gene HNF1B and diabetes mellitus.