AbstractLabile sex expression is frequently observed in dioecious plants, but the underlying genetic mechanism remains largely unknown. Sex plasticity is also observed in manyPopulusspecies. Here we carried out a systematic study on a maleness-promoting gene,MSL, detected in thePopulus deltoidesgenome. Our results showed that both strands ofMSLcontained multiplecis-activating elements, which generated long non-coding RNAs (lncRNAs) promoting maleness. Although femaleP. deltoidesdid not have the male-specificMSLgene, a large number of partial sequences with high sequence similarity to this gene were detected in the female poplar genome. Based on sequence alignment, theMSLsequence could be divided into three partial sequences, and heterologous expression of these partial sequences inArabidopsisconfirmed that they could promote maleness. Since activation of theMSLsequences can only result in female sex lability, we propose that MSL-lncRNAs might play a role in causing sex lability of female poplars.