a, A small fraction of BT474 cells enter into a reversible, quiescent, drug-tolerant persister state in response to 9 or more days of treatment with 2 μM lapatinib. Scale bars, 100 μm. b, Small-molecule inhibitor screen in which pre-derived BT474 persister cells were treated with 1 μM inhibitors while maintained in 2 μM lapatinib. c, Small-molecule inhibitor counter-screen in parental BT474 cells. Images in a are representative of two independent experiments. Data in b and c are from one biologically independent sample from a single screening experiment.