ICMT (isoprenylcysteine carboxyl methyltransferase) is a protein transferase responsible for the isoprenylation modification of proteins near the cell membrane. It is encoded by a single gene and is present on the membranes of the endoplasmic reticulum and Golgi apparatus. ICMT plays a crucial role in cell signaling, protein localization, and cell cycle regulation, particularly in the post-translational modification of Ras proteins and other small GTPases. The specific expression and activity of ICMT are vital for maintaining normal cellular signaling. Mutations or aberrant expression of ICMT are associated with the development of diseases such as cancer, neurodegenerative disorders, and developmental abnormalities.