Plant viruses come in many shapes and sizes: for example, the plant virus Tobacco mosaic virus (TMV) measures 300x18 nm in size; it forms a hollow rod.[27] The plant virus Potato virus X (PVX) forms flexible filaments of 515x13 nm.[28] The following viruses have an icosahedral symmetry and measure between 25-30 nm: plant virus Cowpea mosaic virus (CPMV),[29] bacteriophage Qbeta and mammalian adeno-associated virus (AAV).[30] These are just some examples, many different viruses are being engineered and studied for their potential applications in medicine, some examples of plant viruses include Cowpea chlorotic mottle virus, Red clover necrotic mottle virus, Physalis mosaic virus, Papaya mosaic virus.