Comment on "Increased MKK4 abundance with replicative senescence is linked to the joint reduction of multiple microRNAs".

Marasa et al. (Research Article, 27 October 2009, DOI: 10.1126/scisignal.2000442) reported that the human kinase p38-regulated/activated protein kinase (PRAK) was phosphorylated on residue Ser(93) in senescent cells. We have been unable to detect phosphorylation at this site with the antibody that they used, and the commercial supplier of this antibody has discontinued its availability, which casts doubt on whether this residue of PRAK is phosphorylated.