CUBN (Cubilin) is a multifunctional receptor protein involved in the absorption and transport of key nutrients, including vitamin B12 and various proteins such as albumin. It plays a vital role in the kidneys and intestines, where it helps reabsorb filtered proteins and supports nutrient uptake. In the kidney, CUBN works alongside megalin to maintain protein balance, while in the gut, it is essential for the efficient transport of vitamin B12, supporting normal metabolism and cellular function.