Tartrate-resistant acid phosphatase is a 35 kDa glycosylated monomeric metalloenzyme expressed in mammals. It is highly expressed by osteoclasts, activated macrophages, and neurons. Expression is increased in certain pathological conditions, including leukaemic reticuloendotheliosis (hairy cell leukaemia), Gaucher’s disease, HIV-induced encephalopathy, osteoclastoma and osteoporosis, and metabolic bone diseases. The histochemical identification of hairy cell leukemia via tartrate-resistant acid phosphatase assay has been a standard for over two decades because this antibody labels the cells of hairy cell leukemia with a high degree of sensitivity and specificity.