 Solution

PDGFR is a receptor tyrosine kinase; its key ligand-induced autophosphorylation is on tyrosine residues. Ras-PST25 recognizes phosphorylated serine residues and threonine residues, so it is not the appropriate reagent to detect the characteristic activated PDGFR phosphotyrosines. Use a phosphotyrosine or validated phospho-PDGFR antibody, ideally alongside total PDGFR. The larger receptor may also require a different gel and transfer range, but changing the size range alone would not correct the epitope mismatch.