Haemorrhagic cystitis and polyomavirus‐associated nephropathy are significant clinical entities caused by the BK virus in
solid organ transplant recipients.
Polyomaviruses are unenveloped double-stranded DNA viruses with circular genomes of around 5000 base pairs . The genome is packaged in a viral capsid of about 40-50 nanometers in diameter, which is icosahedral in shape (T=7 symmetry).   The capsid is composed of 72 pentameric capsomeres of a protein called VP1 , which is capable of self-assembly into a closed icosahedron;  each pentamer of VP1 is associated with one molecule of one of the other two capsid proteins, VP2 or VP3 .