After thawing, hepatocyte suspensions were used to collect single hepatocytes into individual 0.2-ml PCR tubes with 2.5 μl of phosphate-buffered saline (PBS) by means of FACS (FACSAria, Becton Dickenson). Selection of the target hepatocyte population was based on the large cell size of hepatocytes (forward-scatter/side-scatter parameters) along with the additional fluorescence staining for DNA content and cell viability. Briefly, bulk hepatocyte suspension samples were prior stained according to the manufacturer’s protocol with the viable DNA-binding dye Hoechst 33342 (Life Technologies) to discriminate cells with a standard diploid chromosome set and LIVE/DEAD Cell Vitality Assay Kit C12 Resazurin/SYTOX Green (Thermo Fisher Scientific) to select viable healthy cells. Typical FACS layout is shown in fig. S1A. Upon sorting, tubes with single cells were frozen on dry ice and kept at −80°C until use.