Using BAC clones from the RPCIB-731 library and sgRNAs with CRISPR/Cas9 technology, a loxP site plus an extra copy of exon 5 (with a T-to-C mutation changing leucine codon 252 (CTG) to proline (CCG) (p.L252P), and an IRES and the GFP reporter gene after the stop codon), intron 5 and exon 6 was inserted downstream of the gene and a second loxP site was inserted into intron 4. The mutation is the equivalent of the p.L265P mutation, in the TIR domain, found in Waldenstromâs macroglobulinemia (WM) and IgM monoclonal gammopathy of undetermined/uncertain significance (IgM MGUS) patients. The mutated exon and the GFP gene will only be expressed after cre-mediated deletion of the floxed endogenous exons 5 and 6. The FRT site flanked neomycin resistance gene cassette that was inserted between the endogenous gene and duplicate exon 5, was removed through subsequent flp-mediated recombination.