Functional B-cell reconstitution requires engraftment of an undifferentiated precursor. The stippled area represents the normal range for 9 wild-type CBA/J mice, and xid controls (5 mice) are denoted by (⧫). All mice were exposed to a sublethal dose of 450 rad and then transplanted with 0.5 × 10⁶ cells, of which 5% were normal. (○) Mice that received unsorted wild-type cells; (□) mice that received the uncommitted, B220⁻/CD19⁻, wild-type cells; (▵) mice that received committed B220⁺/CD19⁺wild-type B cells; and (◊) mice that received the pooled fractions of committed and uncommitted wild-type cells. Each experimental group contained 5 mice.