A poly-A tail is a stretch of adenine nucleotides added to the 3' end of a messenger RNA (mRNA) molecule during the process of transcription. This modification helps stabilize the mRNA and protects it from degradation by enzymes in the cell. It also plays a crucial role in the export of mRNA from the nucleus to the cytoplasm, where protein synthesis occurs. In addition to stability and transport, the poly-A tail is important for the initiation of translation, the process by which proteins are made. The length of the poly-A tail can influence the lifespan of the mRNA and the efficiency of protein production, making it a key factor in gene expression regulation.