To explore whether growth arrest of treated cells coincides with reduced host cell RNA synthesis or overall cell protein biosynthesis, we next assessed the effect of JMN3-003 on host mRNA and protein production. Relative levels of three signature host mRNAs with short half lives, MCL1, ASB7 and MKP1 [31, 32] , were determined by real time PCR after incubation of cells in the presence of different JMN3-003 concentrations ranging from 0.01 to 10 mM. In all cases, mRNA levels of JMN3-003exposed cells were similar to those of the vehicle-treated references, while exposure to Actinomycin D, which blocks RNA synthesis through arrest of the transcription initiation complex [33] , resulted in a major reduction in relative mRNA levels (figure 5A).