ABCG2 and cancer: Several factors are implicated in chemoresistance of TNBC which include cancer stem cells and the pathways that regulate cancer stem cells such as Hedgehog, Wnt/Beta Catenin, Notch and TGF-Beta, deregulation of the apoptotic machinery, alteration in the tumour microenvironment such as hypoxia, the role of ATP-binding cassette (ABC) transporters in drug efflux, microRNA and modulation of various signalling pathways, NF-kB, PTEN/PI3K/AKT/mTOR; JAK/STAT and receptor kinase [7].