Papillary thyroid cancer (PTC) is the commonest endocrine malignancy, though no precursor lesion has been identified yet. Increased expression of cyclin D1 and amplification of its gene along with immunolabelling of HBME-1 in areas showing cytological features of PTC within follicular adenomatoid nodules suggest that these areas could correspond to a precursor lesion of follicular variant of PTC. Our study suggests that the progress of malignancy can be tracked by assessing expression of proliferative proteins like Ki67, pHH3 and cyclin D1, and that apoptosis inhibition by bcl-2 protein can further amplify the role of these proteins in tumour progression. Different mutations in the mitogen-activated protein kinase (MAPK) pathway play distinct roles in the growth and invasion of thyroid cancer cells. We have outlined a possible role of MAPK in the development of PTC, especially that of extracellular regulated kinases (ERK) and p38. MAPK activation may be linked to cyclin D1 expression. (MED - Sciences médicales) -- UCL, 2016