Immunohistochemistry plays a pivotal role in confirming the diagnosis, as these tumors typically express markers such as the epithelial membrane antigen (EMA, a sensitive and specific marker for meningiomas), vimentin (an intermediate filament protein expressed in mesenchymal and glial cells), and progesterone receptor (PR, a characteristic feature of meningiomas). The gene discussed is PGR; the disease is meningioma.