This is a 4.4kDa 37-amino acid antimicrobial peptide called beta-Defensin-4 (hBD-4) having a beta sheet with three intramolecular disulfide bonds. Its expression is inducible in primary keratinocytes but not from healthy human skin. High levels of hBD-4 were detected in testis and in gastric antrum and low levels were found in neutrophils and in the epithelia of the thyroid gland, lungs, uterus and kidney. It can be stimulated by heat-inactivated P. aeruginosa or S. pneumoniae in human respiratory epithelial cells. In addition, it's expression is increased after exposure to PMA and bacteria but not IL-1alpha, IL-6, IFN_gamma or TNF-alpha, which are believed as common inducers of hBDs. Like hBDs-1 and 2, hBD-4 is salt-sensitive in eliciting its antimicrobial activity. With lysozyme it shows a synergistic killing activity of S. carnosus. Similar to hBD-2, it elicits cytokine production by keratinocytes and induces monocyte chemotaxis.