In Part B, 12 patients (8 in the older cohort and 4 in the younger cohort) receive maintenance therapy with cidofovir at the dose established in Part A. Patients who complete 6 months of treatment in Part B may continue therapy until all patients have completed the study. NOTE: For patients who require induction or reinduction, cidofovir is given weekly for 2 weeks before initiating the Part B regimen.