Neonatal and adult T cells are programmed differently. This diagram shows how neonatal T cells are transcriptionally shifted to a more effector-like state in early life by developmentally regulated miRNAs. Importantly, these miRNAs (e.g., miR-29, miR-181, and let-7) do not function as on-off switches, but rather as fine-tuners of gene expression in naive CD4+ and CD8+ T cells at different stages of life. In this way, different miRNAs serve as rheostats for activation (miR-181), effector cell differentiation (miR-29), and metabolism (let-7) in T cells, adjusting various thresholds based upon the need to mount rapid effector or regulatory responses. Abbreviation: TCR, T cell receptor.