With the help of three different commercial antibodies against phosphorylated ERK1/2 and a series of synthetic phospho-peptides employed for competitive blocking experiments, we were able to unequivocally identify and differentiate Tyr- and Thr-monophosphorylated isoforms pT-ERK1, pY-ERK1, pT-ERK2 and pY-ERK2 in cell lysates from stimulated monocytic THP-1 and neuroblastoma SH-SY5Y cell lines and from human PBMCs prepared from whole blood. The gene discussed is MAPK3; the disease is neuroblastoma.