Cysteine codon 651 (TGC) in exon 13 was changed to phenylalanine (TTT) (p.C651F) and a loxP site flanked neomycin resistance gene cassette was inserted into intron 3. The neo cassette was removed through subsequent Cre-mediated recombination. The mutation is located in the PHD domain of the encoded peptide and abrogates its SUMO-E3-ligase activity.