TCF11 (also known as Nrf1 and LCR-F1) is a basic-region leucine-zipper (b-ZIP) transcription factor that is essential during embryonic development. We have carried out expression analysis at a number of developmental stages and find that while there is some localized elevated expression between 8 and 9 days post coitum (dpc), the gene is widely expressed with a constant level of mRNA transcripts detectable in all tissues and at all stages examined. However, this does not reflect the specific nature of a TCF11 mutant phenotype (EMBO J. 17 (1998) 1779) which shows an essential role in foetal liver haematopoiesis. The specificity of TCF11 function may be controlled at a post-transcriptional level including availability of, and specific interaction with, a range of potential heterodimerization partners. We therefore carried out expression analysis of four candidate partner molecules; the three small Maf genes and another bZIP transcription factor found to bind to TCF11 in a two-hybrid screen, ATF4. We show different patterns of expression for the three Maf genes during development with MafG being widely expressed, MafK expressed only later in development and in specific tissues, and no detection of MafF. ATF4 shows evidence of complex regulation during development and shows elevated expression in many of the same sites as TCF11.