Cells

We used human HEK293 (TK1⁺) and hamster A23 cells (TK1^-), each previously validated by low pass sequencing (Khan et al., 2020). Cell fusion was performed as described (Khan et al., 2020). Before fusion, A23 cells were grown in the presence or absence of bromodeoxyuridine (BrdU, 0.03 mg ml^-1). We irradiated the HEK293 cells using 100 (Gray) Gy, with an expected fragment length of 4 Mb, close to the observed. After fusion, cells were plated at a dilution of 1:10 in selective HAT medium (100 μM hypoxanthine, 0.4 μM aminopterin, 16 μM thymidine; Thermo Fisher Scientific^®). RH clones (TK1⁺) were picked at 3 weeks.