On 2014 Aug 26, Anders von Heijne commented:

PML in the posterior fossa remain a clinical and radiological challenge. It is important to keep in mind that JCV infections come in different forms. There are some mutations of JCV that seem to target the cerebellar granule cells specifically, causing granular cell neuronopathy, that at least in early stages only show rapid cerebellar atrophy but with obvious cerebellar symtoms. See for example a recent report on JCV GCN in natalizumab treated MS: Agnihotri et al Neurology. 2014 Aug 19;83(8):727-32

On 2014 Aug 26, Anders von Heijne commented:

PML in the posterior fossa remain a clinical and radiological challenge. It is important to keep in mind that JCV infections come in different forms. There are some mutations of JCV that seem to target the cerebellar granule cells specifically, causing granular cell neuronopathy, that at least in early stages only show rapid cerebellar atrophy but with obvious cerebellar symtoms. See for example a recent report on JCV GCN in natalizumab treated MS: Agnihotri et al Neurology. 2014 Aug 19;83(8):727-32