CDKN1A and Alzheimer disease: When XRCC1 is absent, the p21 gene is not transcribed, as we are able to show using Cre-ad infection of primary Xrcc1 null myoblast cultures, leading to a complete failure to induce p21 expression during differentiation (Figure 4b), suggesting that the post-strand break repair event is critical for gene induction at this loci.