The functional differences between male and female brains commit to the existence of androgen that the testis secretes during the perinatal period. Androgen exerts its action on the brain after conversion to estrogen by brain aromatase. The aromatase appears in some neural nuclei such as in the hypothalamus and amygdala, and has been indicated to be involved in the expression of sexuality by the results of neurobehavioral analyses involving aromatase-knockout mice. We analyzed the brain-specific promoter in order to clarify the control mechanism for the expression of brain aromatase, which is deeply concerned in the sexual differentiation of the brain. The 202bp upstream region of brain-specific exon 1 contains at least three kinds of cis-acting elements, Arom-Aalpha, -Abeta and -B. In particular, the binding activities as to the Abeta sequence show a tissue-specific pattern. Gel shift analysis revealed that the Abeta binding factor recognizes the TTGGCCCCT sequence. Abeta binding activity is detectable at the perinatal stage, but is undetectable at the adult stage in the brain. Furthermore, a protein which binds to the Abeta sequence was purified from the fetal mouse brain. The molecular mass of the Abeta binding protein was estimated to be 49kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.