XRCC4 and immunodeficiency disease: Thus, the residual activity of LIG4 in context of complete XRCC4 deficiency could explain viability and the absence of clinical immunodeficiency in humans, due to a sufficient level of V(D)J recombination to maintain the diversity of the immune repertoire, but with MPD due to a lack of DSB repair in progenitor cells, leading to cycle cell arrest during embryogenesis.