PNP inhibition in red blood cells during therapy. Red blood cells from patients (all 8 patients; except patient no. 6) were isolated from whole blood, starting day 0 to day 4 (n = 7) and day 27 (n = 5), and the cells were processed to determine the level of PNP and its inhibition after start of therapy as described in “Measurement of PNP inhibition.” Data were fitted with the use of the nonlinear rectangular hyperbola. PNP assay was done in quadruplicate, and data are mean ± SD. The average specific activity of PNP was 3.7 mU/min where 1 mU is defined as the amount of PNP crude lysate that catalyzes the phosphorolysis of 1μmole of inosine per minute under standard assay condition.