Residues 17-29 of the LL-37 peptide, also known as FK-13. FK-13 has near-similar anti-microbial and anti-cancer properties to LL-37. This core fragment also contains part of the LL-37 actin binding domain and can associate weakly with actin, actin binding protects this fragment from protease degradation.

LL-37 is a member of the large cationic family of anti-microbial peptides called cathelicidins which have broad-spectrum anti-microbial activity and are expressed in many species. The only cathelicidin found in humans is LL-37; this is produced in epithelial cells, by proteolytic cleavage from the C‑terminal of the hCAP-18 protein. LL-37 can be processed into several different forms of anti-microbial peptides. As well as its anti-microbial properties LL-37 also has anti-cancer properties and regulates many aspects of the innate immune system; overexpression of LL-37 has been linked to autoimmune diseases such as asthma and psoriasis