Three distinct pathways for retroviral assembly. C-type viruses and lentiviruses appear to assemble the internal structures of their particles concurrently with envelopment at the plasma membrane. IAPs are similar except that they bud exclusively into internal membranes. In contrast, D-type and B-type viruses assemble immature particles in the cytoplasm prior to envelopment at the plasma membrane. Spumaviruses, such as HFV, also assemble immature proteins in the cytoplasm but do not undergo an obvious maturation step after budding.