Acute myeloid leukemia 1 (AML1: or runt-related transcription factor, RUNX1) encodes the DNA binding subunit of the heterodimering transcription factor complex PEBP2 (CBF), which plays an essential role for definitive hematopoiesis. Transcription of AML1 is controlled by two distinct promoter regions, which results in the generation of the respective AML1b and AML1c isoforms. Here we report the isolation of the mouse homologue of human AML1c, whose unique N-terminus is 100% identical at the amino acid level to its human counterpart and 63 and 37% identical to the respective family members AML2 and AML3. Semiquantitative RT-PCR assay on mouse embryonic stem cell clones during in vitro differentiation and Northern blot analysis of a mouse embryo revealed that AML1b is expressed in undifferentiated ES cells and upregulated in the early developmental stage, in contrast to the gradual upregulation and steady maintenance of AML1c expression during embryogenesis. In addition, maintenance of AML1c expression depended on the presence of active AML1 allele(s) while that of AML1b did not. Thus, these two AML1 isoforms driven by their respective promoters are differentially expressed and are likely to have distinct functions in early hematopoietic development.