NGAL, also known as lipocalin 2, LCN2, and siderocalin, is a small molecule composed of 178 amino acids. It belongs to the lipocalin superfamily of 20 structurally related secreted proteins. Members of the lipocalin family share a highly conserved fold with an eight stranded antiparallel β barrel, and act as a transporter, carrying small molecules to specific cells. NGAL occurs predominantly in a monomeric form, with a small percentage occurring as a dimer or trimer. It is a critical component of innate immunity because its ability to bind iron-siderophore complexes limits bacterial growth. Mouse NGAL is detected in lung, spleen, uterus, vagina and epididymis. In addition to its role in innate immunity, it is a promising biomarker for acute kidney injury that is beginning to be used in clinical practice in addition to research studies.