PCNA (Proliferating Cell Nuclear Antigen) is a 36 kDa protein also known as cyclin. In early S phase, PCNA has a very granular distribution and is absent from the nucleoli. At late S phase, PCNA is prominent in the nucleoli. There are two forms of PCNA protein present in cells, a soluble form sensitive to organic fixation and not involved in replication, and the other form is insoluble and is associated with ongoing DNA synthesis.