Transcription factors of the basic helix-loop-helix family such as Mash2 are essential for adequate differentiation of the trophoblast. Disruption of the Mash2 gene leads to early intrauterine death caused by placental insufficiency with an absent spongiotrophoblast and an underdeveloped chorion. The aim of the present study was to analyze the structure of the murine Mash2 gene, to screen a broad spectrum of organs for its expression, and to investigate placental Mash2 expression at different gestational ages. The RNase protection assay identified, in addition to the postulated Mash2 mRNA, two unexpected Mash2 transcripts that could be confirmed by a 5′ rapid amplification of cDNA ends. However, all three transcripts were detectable exclusively in murine placenta and not in other organs, such as the ovary, uterus, skin, lung, femur, skeletal muscle, kidney, skull, adrenal gland, tongue, stomach, spleen, skin, testis, or pancreas. Sequence analysis disclosed an additional transcription start site upstream of exon 2. Placental Mash2 mRNA is measurable at all investigated stages of gestation. After its initial detection on Day 8.5 postcoitum (p.c.; set to 100%; 100.0% ± 28.4%), the Mash2 mRNA concentration increases significantly and reaches a maximum of 812.0% ± 69.7% on Day 12.5 p.c. The second half of gestation is marked by a more than 8-fold Mash2 decrease by Day 18.5 p.c. (77.0% ± 28.4%). A 36.9% ± 4.7% level of placental Mash2 mRNA is measurable at term.