FOLR1 and lung adenocarcinoma: However, FOLR1 is also expressed at high levels (1–3 million receptors/cancer cell) on approximately 74 to 85% of lung adenocarcinomas.18–20 Increased FOLR1 expression on neoplastic cells likely arises as a means to compete for low serum folate concentrations (≈ 2 × 10−8 M) as folate is essential to support the increased DNA synthesis machinery in rapidly dividing cancer cells.