As the Mcu knockout allele was generated using blastocysts of C57BL/6 mice17, we probed the FVB/NJ mice backcrossed for the Mcu knockout allele for a well-known polymorphism that causes age-related hearing loss (AHL) in C57BL/6 mice—the Cdh23ahl allelic variant23. The gene discussed is MCU; the disease is acute hemorrhagic leukoencephalitis.