C9orf72-related amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD) has proven difficult to model in mice. Liu et al. (2016) reported a bacterial artificial chromosome (BAC) transgenic mouse displaying behavioural, motor and pathological abnormalities. This was followed by multiple laboratories independently refuting and confirming phenotypes. A proposed explanation centred on the use of different FVB background lines (from The Jackson Laboratory and Janvier Labs). We studied C9orf72 BAC mice on both backgrounds and found significantly elevated levels of dipeptide repeat proteins, but no evidence of a transgene-associated phenotype. We observed seizures and a gradual decline in functional performance in transgenic and non-transgenic mice, irrespective of genetic background. The phenotype was in keeping with the so-called ‘space cadet syndrome’. Our findings indicate that the differences previously reported are not due to C9orf72 status and highlight the importance of using genetic backgrounds that do not confound interpretation of neurodegenerative phenotypes
