Homologous recombination incorporated serine to alanine substitutions in the MAPK-consensus phosphorylation sites (p.S187A, p.S195A, p.S206A, p.S214A) and two conserved phosphoserines in exon 3 (p.(S209_S210delinsAA)). Western blot analysis of MEF nuclear extracts demonstrated the mutant protein was expressed at similar levels as that of wild-type. (J:90769)