Secreted alkaline phosphatase (SEAP) is commonly used as a reporter of gene expression. Compared to other conventional intracellular reporters such as chloramphenicol acetyltransferase (CAT) and firefly luciferase, SEAP has the advantage of being secreted from transfected cells into the culture medium. SEAP activity in the culture medium is directly proportional to changes in intracellular concentrations of SEAP mRNA and protein. In addition, the kinetics of gene expression can be studied using the same cultures by repeatedly collecting culture medium at different time points. The intact cells can be used for further analysis of RNA or protein expression.

SEAP activity was first measured using the chromogenic alkaline phosphatase substrate p-nitrophenyl phosphate (pNPP). Today, the most sensitive SEAP assays employ chemiluminescent alkaline phosphatase substrates such as the 1,2-dioxetane CSPD. Chemiluminescent detection of SEAP is fast, easy to perform, and sensitive.