RET and thyroid gland papillary carcinoma: Using TPC1 cell line system, a papillary thyroid carcinoma cell line in which p90RSK is activated by oncogenic activation of upstream MAPK pathway due to RET/PTC1 rearrangement [35], we verified that endogenous p90RSK is able to phosphorylate MDM2 on S166 and that pharmacological inhibition of p90RSK with increasing concentrations of BI-D1870 (4, 10 μM) determines a progressive dephosphorylation of MDM2 on S166 (Figure 3B, supplementary Figure S3B).