The double-stranded RNA-activated protein kinase, PKR, is a ubiquitously expressed serine/threonine protein kinase that plays a key role in the innate immunity response to viral infection in higher eukaryotes and has also been implicated in several cellular signal transduction pathways. The dsRNAs-mediated activation leads to autophosphorylation of PKR and allows the kinase to phosphorylate its natural substrate, the α subunit of initiation factor eIF2, resulting in rapid inhibition of translation and suppression of virus spread. PKR also has been implicated in regulating other cellular functions such as differentiation, transcription, signal transduction, cell growth and apoptosis in the event of virus infection and other forms of cellular stress. It was reported that the activation of PKR in adipose and liver tissue is caused by obesity. In the absence of PKR, metabolic deterioration due to excess energy or nutrition is alleviated. These findings demonstrate that PKR is an important component of inflammation complex that responds to nutrients and organelle dysfunction.<br />The phosphorylation status of PKR at threonine 451 is shown to be a nice marker of the activation of PKR <em>in vitro</em> as well as<em> in vivo</em>.