(A) Analysis of 60 lentiviral vector batches produced using bluebird bio's manufacturing process showed a linear correlation between harvest infectious titers and final product titers, regardless of the construct or scale. The correlation was strongest at the 2L scale. (B) Harvest titers correlated well with p24 concentrations, which can estimate total particle concentrations. Estimated particle-to-infectivity ratios for 6 batches averaged 84 and ranged from 70-103. (C) Process yields averaged 37% and were consistent, with 75% of batches between 30-50%. Most batch failures were due to specific constructs producing low titers.