Evolutionary conserved alternative 5' UTR-containing 1st exon 1b and associated promoter (Rr56937) were targeted with sgRNAs (targeting GGAGATCTGGGTAGTCCTCG and GCTTTTCATACTATAGTGAG) using CRISPR/Cas9 technology, resulting in a 3064 bp deletion (chr2:66237911-66240974 (GRCm39)).