The folate receptors consist of five members, (alpha), (beta), (gamma), (gamma)' (which is produced by alternative splicing) and (delta). (beta)-FR is attached to the membrane by a GPI anchor and is expressed in malignant tissues of epithelial and nonepithelial origin. (gamma)-FR is expressed in tissues of hematopoietic origin, such as spleen, thymus and bone marrow. The expression pattern of (delta)-FR is elusive, which suggests that it is highly restricted both spatially and temporally. The (alpha) isoform of the folate receptor is used as a highly selective tumor marker and may be targeted for the delivery of therapeutic compounds to tumor cells by coupling to derivatives of folic acid.