Heat shock proteins (HSPs) play oncogenic roles in human tumours. We reported a somatic inactivating mutation of HSP110 (HSP110DE9) in mismatch repair-deficient (dMMR) cancers displaying microsatellite instability (MSI) but did not assess its impact. We evaluated the impact of the Hsp110DE9 mutation on tumour development and the chemotherapy response in a dMMR knock-in mouse model (Hsp110DE9(KI)Msh2(KO) mice). The effect of the Hsp110DE9 mutation on tumorigenesis and survival was evaluated in Msh2(KO) mice that were null (Hsp110(wt)), heterozygous (Hsp110DE9(KI/+)), or homozygous (Hsp110DE9(KI/KI)) for the Hsp110DE9 mutation by assessing tumoral syndrome (organomegaly index, tumour staging) and survival (Kaplan-Meier curves). 5-Fluorouracil (5-FU), which is the backbone of chemotherapy regimens in gastrointestinal cancers and is commonly used in other tumour types but is not effective against dMMR cells in vivo, was administered to Hsp110DE9(KI/KI), Hsp110DE9(KI/+), and Hsp110(wt)Msh2(KO) mice. Hsp110, Ki67 (proliferation marker) and activated caspase-3 (apoptosis marker) expression were assessed in normal and tumour tissue samples by western blotting, immunophenotyping and cell sorting. Hsp110(wt) expression was drastically reduced or totally lost in tumours from Msh2(KO)Hsp110DE9(KI/+) and Msh2(KO)Hsp110DE9(KI/KI) mice. The Hsp110DE9 mutation did not affect overall survival or tumoral syndrome in Msh2(KO)Hsp110DE9(KI/+) and Msh2(KO)Hsp110DE9(KI/KI) mice but drastically improved the 5-FU response in all cohorts (Msh2(KO)Hsp110DE9(KI/KI): P5fu = 0.001; Msh2(KO)Hsp110DE9(KI/+): P5fu = 0.005; Msh2(KO)Hsp110(wt): P5fu = 0.335). Histopathological examination and cell sorting analyses confirmed major hypersensitization to 5-FU-induced death of both Hsp110DE9(KI/KI) and Hsp110DE9(KI/+) dMMR cancer cells. This study highlights how dMMR tumour cells adapt to HSP110 inactivation but become hypersensitive to 5-FU, suggesting Hsp110DE9 as a predictive factor of 5-FU efficacy.