TRA-2-54, also known as Liver/Bone/Kidney Alkaline Phosphatase (ALP or AP), is a tissue non-specific isoform of alkaline phosphatase-related antigen. This antigen is highly expressed on undifferentiated human embryonal carcinoma (EC) and embryonic stem (ES) cells, and is also expressed by liver, bone and kidney cells. It has been reported that the expression level of TRA-2-54 is decreased upon differentiation; therefore measurement of TRA-2-54 expression can be used to monitor the differentiation status of pluripotent stem cells.