MUC1 and breast adenocarcinoma: After we had labeled MUC1-overexpressing MCF-7 cells (human breast adenocarcinoma cell line) with AptMUC1–Au NPs/GO, we used LDI-MS to monitor Au cluster ions ([Aun]+; n = 1–3), resulting in the detection of as few as 100 MCF-7 cells.