CRISPR/Cas9 was used to insert a single base pair (A on negative strand) between Chr5:71,014,638-639 to correct the spontaneous deletion of an A in intron 4 that is inherent in C57BL/6J and BXD40. This restores the allele to the "wild-type" form as found in C57BL/6NJ, C57BL/6ByJ, C57BL/6EiJ, DBA/2J and others. (J:280223)