The study investigates the effects of microRNAs (miRNAs) on target mRNA levels and protein production in mammalian cells. Using ribosome profiling, the researchers measured the impact of miRNAs on both mRNA levels and protein production. For both ectopic and endogenous miRNA regulatory interactions, the results show that lowered mRNA levels account for most of the decreased protein production, indicating that miRNAs predominantly act to reduce mRNA levels rather than directly inhibit translation. The study also found that the molecular consequences of miRNA regulation are consistent with the mRNA-destabilization scenario, where miRNAs cause mRNA destabilization through de-adenylation and subsequent degradation, rather than through translational repression. These findings provide insights into the mechanisms of miRNA-mediated gene regulation and suggest that changes in mRNA levels closely reflect the impact of miRNAs on gene expression.The study investigates the effects of microRNAs (miRNAs) on target mRNA levels and protein production in mammalian cells. Using ribosome profiling, the researchers measured the impact of miRNAs on both mRNA levels and protein production. For both ectopic and endogenous miRNA regulatory interactions, the results show that lowered mRNA levels account for most of the decreased protein production, indicating that miRNAs predominantly act to reduce mRNA levels rather than directly inhibit translation. The study also found that the molecular consequences of miRNA regulation are consistent with the mRNA-destabilization scenario, where miRNAs cause mRNA destabilization through de-adenylation and subsequent degradation, rather than through translational repression. These findings provide insights into the mechanisms of miRNA-mediated gene regulation and suggest that changes in mRNA levels closely reflect the impact of miRNAs on gene expression.