We selected transglutaminase-2 (TGM2), which forms isopeptide bonds between glutamines and ε-amino groups of lysines (11), as the candidate because it is expressed in human PDA, CRC, and breast cancers (12) and is present in lysates of the SKBR3 and Caco2 cell lines (SI Appendix, Fig. S3). Here, TGM2 is linked to breast carcinoma.