Cellular assays. HeLa (DSMZ; ACC 57) and A549 (DSMZ; ACC 107) cell lines were purchased from the DSMZ. TrxR knockout and reference mouse embryonic fibroblasts (MEF) were a kind gift from Marcus Conrad. All cell lines are tested regularly for mycoplasma contamination and only mycoplasma negative cells are used in assays. Cells were grown in Dulbecco's modified Eagle's medium (DMEM; PAN Biotech) at 37 °C under 5% CO2 atmosphere, supplemented with 10% FBS, penicillin (100 U/mL), streptomycin (100 µg/mL) and Na2SeO3 (100 nM) unless stated otherwise. Selenium supplementation / starvation changes were always followed by at least 7 days of culturing the cells in the new media before performing any experiments. For assays, cells were typically seeded in 96well plates (black, Fluotrack, high binding; Greiner) in 100 µL medium, probes were added (1% DMSO final) using a D300e digital dispenser (Tecan), and timecourse fluorescence measurements were conducted using a BMG FluoStar Omega platereader (ex 355bp20, em 520lp). Cells were kept at 37 °C under 5 % CO2 atmosphere between measurements.