The effects of overexpression of the wt and mutant hnRNP A1 on syncytium formation and virus production We ®rst assessed the effects of hnRNP A1 overexpression on the morphological changes induced by MHV-A59 infection using several different clones of DBT cell lines. Virus infection was performed at a multiplicity of infection (m.o.i.) of 0.5 to detect the subtle morphological differences among the different cell lines. Syncytia appeared at~7 h post-infection (p.i.) in DBT-VEC cells and~1 h earlier in DBT-A1 cells. At both 8 and 14 h p.i., syncytia were signi®cantly larger and more spread out in DBT-A1 cells than those in DBT-VEC cells ( Figure 2A ). Similar differences were observed with two additional clones of DBT-A1 cells (data not shown). In contrast, no syncytium was observed in three different clones of DBT-A1DC cells, even at 14 h p.i. At 24 h p.i., almost all DBT-A1 cells detached from the plate, but~10±20% of DBT-VEC cells still remained on the plate (data not shown). Remarkably, there was no sign of syncytium formation in DBT-A1DC cells until 24 h after virus infection, when the overall morphology of the cells was similar to that of DBT-VEC cells at 7 h p.i. (data not shown). All of the DBT-A1DC cells were eventually killed at~48 h p.i., suggesting that the inhibition of viral replication was not a result of the disruption of the MHV receptor. Correspondingly, virus production from these cell lines was signi®cantly different. Between 6 and 14 h p.i., virus production from DBT-A1DC cells was 100-to 1000-fold less than that from DBT-VEC and DBT-A1 cells ( Figure 2B ). DBT-A1 cells produced twice as many viruses as those from DBT-VEC cells during that time period.