siRNA, or 'short interfering RNA', comprises two complementary RNA strands, 19-21 nucleotides each, with TT or dTdT overhangs. Longer double-stranded RNA can trigger immune responses and degradation. Dicer is believed to add these overhangs during processing. Target cleavage likely occurs in the middle of antisense sequences, preserving the middle bases. Synthetic siRNAs are delivered into cells via electroporation or lipid delivery to initiate RNAi. Once inside, the strands separate, releasing the antisense strand. With the help of a protein called RISC, it binds to a complementary sense sequence on the mRNA molecule. If the base-pairing is perfect, the mRNA is destroyed.