DUX4 and viral infectious disease: In addition, the transient co-expression of ICP0 and ICP4 is sufficient to induce DUX4 expression in the absence of viral infection (Fig. 2D), whereas an E3-ubiquitin-ligase deficient mutant of ICP0 (ICP0 FXE) was not able to induce expression of DUX4 when coexpressed together with ICP4 (Fig. 2D).