MYC and infection: As shown in Figs. 4a, b, 10 days after infection, c-Myc-expressing PEFs (i.e., piCLCs) exhibited the typical short spindle-like or polygonal-like morphology of cultured pPr Ch (Fig. S1B and Fig. 4b, Supplementary information), with intense toluidine blue and alcian blue staining compared with vector-expressing PEFs (Fig. S5, Supplementary information), which had a fibroblast-like appearance, indicating proteoglycan production in induced cells.