This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Acute Lymphoblastic Leukemia, Subject name: Pathology
Which of the following is not expressed in majority of cases of pediatric B-cell acute lymphoblastic leukemia? 
Select and explain. 
A. Terminal deoxynucleotidyl transferase (TdT), B. CD19, C. CD10, D. CD7
D: CD7
Answer: D: CD7 
Explanation: CD19 is a pan B-cell marker. CALLA (common acute lymphoblastic leukemia antigen) is CD 10. CD45 is known as leukocyte common antigen (LCA). Terminal deoxynucleotidyl transferase (TdT), a specialized DNA polymerase that is expressed only in pre-B and pre-T lymphoblasts, is positive in more than 95% of cases. CD7 is commonly expressed in T-cell ALL.