IL-2 or IL-15 maintain but do not expand NK cells from IκBα°ϵ° chimeras. Splenocytes (2 × 10⁵) from WT (Rag° mice, triangles) or IκBα°ϵ° chimeras (circles) were cultured in complete medium with or without IL-2 or IL-15 and percentages of NK evaluated daily. NK cells were rapidly lost in cultures lacking exogenous cytokines (open symbols). IL-2 (or IL-15, •) maintained but did not expand IκBα°ϵ° NK cells (absolute NK cell number was 27 000 on day 5). WT NK cells proliferated vigorously over the same time period (NK cell number was 8 × 10⁵ at day 5).