IRF4 and Bloom syndrome: In studies of the molecular basis of the NOD2 defect underlying BS we showed first that, as discussed above, cells from mice with NOD2 mutations in the NOD2 NBD resulting in signaling via RIPK2 and in activation of NF-κB also manifested a defect in the ability to upregulate expression of IRF4, a factor previously identified as necessary for NOD2 cross-regulation (16, 21, 25).