CCND1 and adenoma: Based on our previous study and the results from the present study that demonstrated a positive association between CCND1 expression and adenoma weight5, we speculate that under-expression of tumor suppressor gene products (p15 and p16) and overexpression of oncogene (cyclin D1) may explain both the mechanism of parathyroid tumorigenesis and the severity of disease expression (higher adenoma weight and PTH level, and a more severe and symptomatic form of PHPT)7.