ABCG2 and cancer: Moreover, MLN4924 reduces the percentage of SP cells in NPC cells in a dose-dependent manner, while the population of SP cells in cisplatin treated S18 cells reached up to 90%.34 Wang’s study found that the SP assay was a viable method to identify cancer stem cell-like cells in human NPC cell lines.35 SP assay is based on the ATP-binding cassette (ABC) half transporter member 2 of G family protein (ABCG2), which can efflux Hoechst 33342 out of cells.