Kinetics of c-kit downregulation, TdT expression and acquisition of CD45R in B-lineage differentiation . Our studies suggest that TdT expression is initiated as c-kit downregulation begins in Lin⁻ cells, with progressive loss of c-kit during B-lineage differentiation and that CD45R expression is initiated during the transition from c-kit^Lo to c-kit⁻ with many cells losing c-kit before acquiring CD45R. However, this loss is not precisely synchronized with maturity and a minority of B-lineage cells retain or reacquire c-kit during the early pro-B stage.