Until recently, the basic leucine zipper transcription factor E4BP4 (also known as NFIL3) was of little interest to immunologists, being best known for its role in regulating circadian rhythm in chick pineal gland. However, characterisation of E4bp4^–/– mice, independently generated in four different laboratories, has revealed roles for E4BP4 in diverse haematopoietic lineages. E4BP4 is essential for the development of NK cells and CD8α⁺ conventional dendritic cells, and is also involved in macrophage activation, polarisation of CD4⁺ T cell responses and B cell class switching to IgE. Here, we discuss the role of E4BP4 as a regulator of the immune response and highlight future questions for the field.