ABCG2 and adenoma: We also noticed that many types of secretion systems could be involved in the advanced level of adenoma–carcinoma sequence: the type 3 secretion ESPL1 and ESPX1 (type III secretion systems are used by pathogenic species to inject toxins into attacking immune cells [31,32]), and general secretion system D, F, K (general secretion pathway can translocate the small-signal sequence-containing proteins into the periplasm which can be targeted then to ABC transporter/TolC-based export system such as the enterotoxins produced by E. coli [33]).