From the FDA:
Genentech has informed healthcare professionals of revisions to prescribing information for Rituxan regarding a case of progressive multifocal leukoencephalopathy (PML) leading to death in a patient with rheumatoid arthritis who received Rituxan in a long-term safety extension clinical study. PML is a brain infection and had already been noted on Rituxan's label as a possible risk.
The patient developed a JC virus infection with resultant PML and death 18 months after taking the last dose of Rituxan.
Healthcare professionals treating patients with Rituxan should consider PML in any patient presenting with new onset neurologic manifestations. Additionally, consultation with a neurologist, brain MRI and lumbar puncture should be considered as clinically indicated.
You can find the Dear Doctor letter here. (PDF)