The Igκ-LPETG mouse model. (A) The κ–LC constant region was targeted using CRISPR-Cas9 to introduce a SrtA motif at the κ C-terminus. The OB1 HC^+/+ Igκ-LPETG^+/+ and WT HC Igκ-LPETG^+/+ mouse models have B cells of IgG1 and IgM/D BCRs, respectively. (B) The Igκ-LPETG motif on the BCR can be targeted by any substituent of choice (biotin, organic dyes, proteins, peptides, radioactive isotopes, DNA) using SrtA. SrtA cleaves between the T and G in the LPETG motif to produce an acyl-enzyme intermediate. The glycine N-terminus of the GGG motif on the probe can then initiate a nucleophilic attack resulting in a covalent (peptide) linkage of the probe to the BCR.