The coding region in the last exon was replaced with the same fragment coding for a cleavage-resistant form with p.R135M (c.404G>T) and p.R137L (c.410G>T) mutations, and a loxP site flanked neomycin resistance gene cassette was inserted downstream of the gene. The neo cassette was removed through subsequent Cre-mediated recombination. The mutations, close to the encoded peptide cleavage site, affect proteolytic cleavage of the prodomain that is necessary to form the mature peptide. They mimic the human tandem SNPs c.374G>T (rs1048220) and c.380G>T (rs1048221).