SLC22A4 is a gene that provides instructions for making a protein involved in transporting various substances across cell membranes. This protein is part of a larger family known as solute carrier proteins, which help move molecules like drugs and metabolites into and out of cells. The SLC22A4 protein is particularly important in the kidneys and immune system, where it plays a role in the elimination of waste products and the regulation of immune responses. Variations in this gene have been linked to certain health conditions, including autoimmune diseases like rheumatoid arthritis.