TdT is a marker to identify existence of cancer in blood cells known as leukemia. Terminal Deoxynucleotidyl Transferase (TdT) is a template-independent DNA polymerase that catalyzes the repetitive addition of deoxyribonucleotides to the 3'-OH of oligo-deoxyribonucleotides and single-stranded and double-stranded DNA. TdT is a highly specific marker for the diagnosis and classification of acute lymphoblastic lymphoma (ALL) or leukemia. The test is used to classify certain Leukemia and lymphomas, normally used to distinguish lymphoblastic from non-lymphoblastic leukemia, diagnose acute lymphoblast leukemia, lymphoid blast crisis of chronic myelogenous leukemia, and lymphoblastic lymphomas.