Still, in a significant number of individuals, the BiTE antibody construct has insufficient activity despite expression of CD19 on leukemic cells.5, 6, 7 Results from a large multicenter phase 2 trial of blinatumomab monotherapy suggest that the tumor burden present at the time of therapy may be one critical determinant for the clinical activity of BiTE antibody constructs.7 However, other determinants of response have yet to be identified. The gene discussed is CD19; the disease is neoplasm.