B220 is a lineage associated– rather than an activation marker in the NK-cell differentiation pathway and defines pre-mNK cells. Guimont-Desrochers et al adoptively transferred various NK-cell subsets and followed their proliferation and phenotype in immunocompetent versus Rag1-deficient recipients treated or not with activating stimuli (known to up-regulate B220 molecules in vitro). While IKDCs remained B220 at early time points without activation (and maintained this expression at later time points after activation), B220⁻ NK cells (expressing or not CD27) failed to acquire B220. The follow-up study of B220⁺ NK cells revealed that this subset differentiated into maturing NK cells over time, losing B220, Ly108, and CD27 while acquiring CD11b and CD43 by day 25.