boss protein (as assayed with bossT:Arus\HRP, in somatic clones in the eye is not properly internalized into endosomes. Delivery of bossT:Arus\HRP to the R8 cell surface is not blocked. Generally embryonic lethal when homozygous, though some survive to larval stages though are thin, translucent and developmentally delayed. They often contain melanotic masses and clumps near the brain. Eye discs show the same pattern of boss staining as for homozygous clones. R7 specification is normal. Garland cells of mutant larvae show no uptake of TR-avidin indicating that the internalisation of endocytic traces is completely blocked in mutant cells. Morphology of garland cells is altered:labyrinthine channels are absent, coated vesicles are absent and there is a dramatic increase in multilamellar lysosomal structures.