Recombinant cells including one or more transiently overexpressed genes encoding a drug transporter protein, wherein the recombinant cell is cryopreserved and activity of the drug transporter protein is detectable in a population of the recombinant cells prior to cryopreservation at an uptake ratio of at least 5. Processes of preparing cryopreserved transiently transfected recombinant cells, including transiently transfecting cells with one or more genes encoding a drug transporter protein and cryopreserving the transiently transfected recombinant cells within 48 hours of transfection. A population of the transiently transfected recombinant cells transiently overexpress the one or more genes encoding the drug transporter protein at a detectable level prior to cryopreserving and the detectable level is an uptake ratio of at least 5.
