Fibrolamellar hepatocellular carcinoma (FL‑HCC) is a variant of hepatocellular carcinoma (HCC) that most commonly affects adolescents and young adults and is associated with an extremely poor prognosis due to the lack of effective chemotherapeutic agents. Mutations in p53 are a common oncogenic driver in HCC but not in FL‑HCC. However, in tumors lacking a p53 mutation, the tumor suppressor activity of p53 has been revealed to be dysregulated in several different cancer types. One mechanism has been attributed to the overexpression of mouse double minute 4 protein (MDM4), a negative regulator of p53, which inhibits the normal functions of p53 including induction of apoptosis and DNA repair. Therefore, restoring the normal function of p53 in cancer cells by targeting MDM4 has become a potential therapeutic strategy. Hence, in the present study the components of the DNA damage response (DDR) pathway were examined; ATM, p53, and MDM4 in FL‑HCC. Seven FL‑HCC tumors along with their adjacent non‑neoplastic hepatic tissues were examined. Ataxia‑telangiectasia mutated (ATM), p53, and MDM4 protein expression was assessed using western blot analysis and cellular localization was determined using immunohistochemistry (IHC). MDM4 mRNA transcript levels were assessed using RT‑qPCR. The present results demonstrated that the DNA damage sensor, ATM, is phosphorylated and localized to the nuclei of tumor cells. While there was a significant increase in total p53 protein in tumor cells, phosphorylated p53 was revealed to preferably localize to the cytoplasmic compartment of tumor cells. Notably, the present results revealed that MDM4 transcript levels were increased in the majority of tumor samples and the nuclear MDM4 levels were significantly increased in tumor tissue compared to their adjacent non‑neoplastic liver tissue. The present results indicated that increased MDM4 expression and nuclear localization may be a potential mechanism for p53 dysregulation in FL‑HCC.