The Antennapedia homeodomain protein ofDrosophilahas the ability to penetrate biological membranes and the third helix of this protein, residues 43–58, known as penetratin (RQIKIWFQNRRMKWKK-amide) has the same trans-locating properties as the entire protein. The variant, RQI KIFFQNRRMKFKK-amide, here called penetratin (W48F,W56F) does not have the same ability.