Proteins can form homomeric complexes with another unit of the same protein or heteromeric complexes with different types. Most protein complexes self-assemble spontaneously via ordered pathways, while some proteins need assembly factors that guide their proper assembly. Despite the crowded intracellular environment, proteins usually interact with their correct partners and form functional complexes. Many viruses self-assemble into a fully functional unit using the infected host cell to produce all the necessary components. The tobacco mosaic virus (TMV) is a classic example of a self-assembling complex of protein subunits and RNA. The TMV self-assembly process is the basis for some of the therapeutic virus particles designed for drug delivery. Some protein complexes self-assemble only due to mutation or disorders. Hemoglobin, the oxygen-carrying protein found in red blood cells, is a tetramer of two alpha and two beta subunits. However, in sickle cell anemia, a single point mutation replaces a hydrophilic amino acid- glutamine with a hydrophobic valine. This creates sticky hydrophobic patches on the opposite sides of the hemoglobin tetramer. In an aqueous cell environment, the hydrophobic patches on different hemoglobin molecules attach, assembling into long and rigid fibers. These fibers change the red blood cells' shape from a spheroid to a crescent shape.