Monoamine oxidase (MAO) A is the major metabolizing enzyme of serotonin (5-hydroxytryptamine, 5-HT) which regulates early brain development. In this study, wild-type (WT) and MAO A(neo) embryonic stem (ES) cell lines were established from the inner cell mass of murine blastocysts and their characteristics during ES and differentiating stages were studied. Our results show that the differentiation to neural cells in MAO A(neo) ES cells was reduced compared to WT, suggesting MAO A played a regulatory role in stem cells neural differentiation.