This family includes the CD20 protein and the beta subunit of the high affinity receptor for IgE Fc. The high affinity receptor for IgE is a tetrameric structure consisting of a single IgE-binding alpha subunit, a single beta subunit, and two disulfide-linked gamma subunits. The alpha subunit of Fc epsilon RI and most Fc receptors are homologous members of the Ig superfamily. By contrast, the beta and gamma subunits from Fc epsilon RI are not homologous to the Ig superfamily. Both molecules have four putative transmembrane segments and a probably topology where both amino- and carboxy termini protrude into the cytoplasm. This family also includes LR8 like proteins from humans, mice and rats. The function of the human LR8 protein is unknown although it is known to be strongly expressed in the lung fibroblasts. This family also includes sarcospan is a transmembrane component of dystrophin-associated glycoprotein. Loss of the sarcoglycan complex and sarcospan alone is sufficient to cause muscular dystrophy. The role of the sarcoglycan complex and sarcospan is thought to be to strengthen the dystrophin axis connecting the basement membrane with the cytoskeleton.