This peptide contains the unique amino-terminal phosphorylation site of Xenopus ADF/cofilin, the LIM kinase (LIMK) phosphorylation site. LIMK1 is a key regulator of the actin cytoskeleton through its phosphorylation of ADF/cofilin at serine-3 for inactivation. This peptide is a fragment of the S3 peptide containing the serine-3 sequence of ADF/cofilin that has been widely used as an effective competitive inhibitor of LIMK1.