To investigate the role of the endogenous human TP53 3′UTR in post-transcriptional p53 regulation, we used a pair of CRISPR/Cas9 guide RNAs to delete the non-essential part of the 3′UTR in HEK293 cells and in the human colon carcinoma cell line HCT116, an established model for investigating p53-dependent functions (Figure 1A, blue, Figure 1—figure supplement 1A). Here, TP53 is linked to colon carcinoma.