A mutation (C to A) was introduced by site-directed mutagenesis. This resulted in an asparagine to lysine change at amino acid residue 617 (N617K). In addition, a floxed TK-Neo selection cassette was inserted in exon 3 after the region encoding the collagenous domain. The presence of the mutation was confirmed by direct sequencing. Transient transfection with cre recombinase removed the floxed Neo-TK selection cassette.