The predominant lymphocytes in DCIS were CD3+ cells either intratumourally (median 1 cell/duct, range 0–50) or in the stroma (median 3 cells/duct, range 0–100) whereas CD4+ cells were the most frequent subtype of T cells (range 0–60 in the stroma and 0–25 in the intratumoural compartment). This evidence concerns the gene CD4 and ductal breast carcinoma in situ.