Summer Research Description: There is a need to find effective therapeutic targets in triple-negative breast cancer (TNBC) due to its aggressive nature. Although TNBC is a molecularly heterogeneous disease, more than 60% of reported cases harbor missense mutations in the p53 tumor suppressor. p53 is a transcription factor that is activated in response to cellular stress and DNA damage, leading to the transcription of a wide variety of target genes, including cell cycle inhibitors, apoptosis effectors, and its own negative regulator, the E3 ligase Mdm2. Our laboratory is exploring the biochemical state associated with mutant p53 using gene editing techniques. Our preliminary data demonstrate that CAL51, a TNBC cell line expressing wild-type p53, undergoes transient p53 accumulation and target gene activation in response to UV-induced DNA damage. Based on previous reports, we hypothesized that an isogenic, homozygous R248W mutant clone of CAL51 would lack the ability to transactivate MDM2 and would thereby display increased baseline p53 levels and a lack of accumulation in response to UV damage. However, we observed that while the mutant cells indeed harbor higher baseline p53 protein levels and lack expression of known target genes, they display additional, transient p53 accumulation upon UV exposure, implying the retention of some regulatory capacity. Based on these findings, we are exploring alternate mechanisms of p53 regulation, with potential relevance to ongoing efforts to therapeutically “re-activate” p53 in cancer.