Jc polyomavirus bk polyomavirus polyomaviridae

Which of the following is the primary cause of progressive multifocal leukoencephalopathy (PML)?

1: BK virus is the primary cause of PML.
2: JC virus is the primary cause of PML.
3: BK virus and JC virus cause PML approximately equally.
4: All viruses in the Polyomaviridae family cause PML approximately equally.