Terminal deoxynucleotiyl transferase (TdT) is a nuclear transcription factor expressed in B and T-cell precursors and cortical thymocytes.  It is expressed in acute lymphoblastic lymphoma (95%), AML (<25%), Merkel cell carcinoma (~50%), hematogones, and thymomas.  It is negative in Burkitt lymphoma.

 

Scattered TdT positive cells in benign lymphoid tissue (e.g. tonsil) is a common finding.

 

References:

Bone Marrow IHC.  Torlakovic, EE, et. al. American Society for Clinical Pathology Pathology Press © 2009.  pp. 167.

 

Sidiropoulos, M., Hanna, W., Raphael, S. J., & Ghorab, Z. (2011). Expression of TdT in Merkel cell carcinoma and small cell lung carcinoma. American Journal of Clinical Pathology, 135(6), 831–838. doi:10.1309/AJCPLCB2Q9QXDZAA 

 

Onciu, M., Lorsbach, R. B., Henry, E. C., & Behm, F. G. (2002). Terminal deoxynucleotidyl transferase-positive lymphoid cells in reactive lymph nodes from children with malignant tumors: incidence, distribution pattern, and immunophenotype in 26 patients. American Journal of Clinical Pathology, 118(2), 248–254. doi:10.1309/L3UW-3AE7-L4LB-3QX3

 

Strauchen, J. A., & Miller, L. K. (2001). Terminal deoxynucleotidyl transferase-positive cells in human tonsils. American Journal of Clinical Pathology, 116(1), 12–16. doi:10.1309/M8V2-DWPB-DVX1-UBPC  

 

Hurford, M. T., Altman, A. J., Digiuseppe, J. A., Sherburne, B. J., & Rezuke, W. N. (2008). Unique Pattern of Nuclear TdT Immunofluorescence Distinguishes Normal Precursor B Cells (Hematogones) From Lymphoblasts of Precursor B-Lymphoblastic Leukemia. American Journal of Clinical Pathology, 129(5), 700–705. doi:10.1309/ANERT51H38TUEC45