Co-culture of naïve mouse trigeminal neurons with the MOR knockout Fadu cell line inhibited neurite outgrowth compared to untreated neurons or those co-cultured with MOR-expressing cancer cells, suggesting that the absence of MOR activity in Fadu cancer cells alters the tumor microenvironment to reduce neurotropism, potentially impacting the development of perineural invasion in vivo [14]. The gene discussed is OPRM1; the disease is neoplasm.