Essential for the formation of matrix-filled interrhabdomeral space: critical for the formation of epithelial lumina in the retina. Acts together with prominin (prom) and the cell adhesion molecule chaoptin (chp) to choreograph the partitioning of rhabdomeres into an open system. Secreted by photoreceptor cells, through the stalk membrane into the interrhabdomeral space (IRS). Although secreted, lacks a canonical signal sequence and contains a predicted transmembrane domain. The discrepancy may be explained by protein cleavage after the transmembrane region. Expressed from the beginning of rhabdomere biogenesis (48 hours after pupal formation), when it decorates the entire photoreceptor apical surface. Expressed during embryonic, larval, and adult stages. Flies exhibit a closed rhabdomere system, rhabdomeres within each ommatidium are fused to each other. Belongs to the EYS family.