IFNA1 and infection: However, even in these ‘IFN-compromised’ Hep2 cells, MuV Enders had delayed growth kinetics since following a low m.o.i., the virus reached titres of 106–107 p.f.u. ml−1 at 4–6 days post-infection (p.i.), whereas in Vero cells [monkey cells which do not produce IFN due to spontaneous gene deletions (Desmyter et al., 1968; Mosca & Pitha, 1986)], it had reached titres of 107 p.f.u. ml−1 by 2 days p.i. Also in contrast with MuV Enders, PIV5 reached titres of 107–108 p.f.u. ml−1 in both ‘IFN-competent’ and ‘IFN-compromised’ Hep2 cells by 2 days p.i. (Fig. 1).