RTEL1 and telomere syndrome: Loss of the p53 C terminus increases p53 activity in mouse embryonic fibroblasts (MEFs) and in most tested tissues2, 3, and p53Δ31/Δ31 MEFs exhibited decreased messenger RNA (mRNA) levels for 4 out of 10 genes implicated in telomere syndromes (Dkc1, Rtel1, Tinf2 and Terf1).