CDK2 and fibrosarcoma: Various other cytoplasmic protein kinases did not affect the FRET/CFP ratio, namely CDK2 (cyclin-dependent kinase 2), CK2α (casein kinase 2 α-subunit), NLK1 (nemo-like kinase, type 1) and GSK3β (glycogen synthase kinase 3β), as well as ERK1, ERK2 (extracellular-regulated kinases 1 and 2) and constitutively active RAF (rapidly accelerated fibrosarcoma; Fig. 2D,E).