IEM analysis defines the SLC35D3-containing structures in G1ME cells as tubulovesicular endosomes. Large, multinucleated G1ME cells that had been doubly infected with GATA-1– and HA-SLC35D3–encoding retroviruses were enriched by fractionation on a BSA density gradient, then fixed and processed for IEM. Ultrathin cryosections were immunogold labeled with anti-HA Abs and 10-nm protein A gold. (A) Label is shown in vacuolar (E) and tubulovesicular structures near the plasma membrane (PM). (B-D) Immunogold-labeled tubulovesicular structures were frequently observed in close apposition to characteristic early endosomal vacuoles (E) with few luminal vesicles (B), near the Golgi (G) complex (C), and near MVBs (D). MVBs themselves were only occasionally labeled. Scale bars indicate 200 nm.