Beethoven deaf mice are deafened by a point mutation (T into A) in the Tmc1 gene at locus 1,235, causing hearing impairment associated with reduced HCs in the inner ear and successfully targeting the Tmc1 gene by the lipid-mediated entry of the Cas9-gRNA complex into the mouse cochlea, resulting in a shift mutation and eventual loss of function due to a random insertion–deletion, which improves the survival of HCs while improved the hearing of mice (Gao et al., 2018). The gene discussed is TMC1; the disease is hearing loss disorder.