In Reply.—We are grateful for the authors' interest in our recent publication on the use of PAX-2 as a marker for metastatic renal cell carcinoma.1 The authors have appropriately pointed out that PAX-2 can be expressed by a subset of lymphocytes, especially B lymphocytes. Therefore, the lymphocyte nuclei may be positive with immunostain for PAX-2.

It has been well known that an antibody often decorates not only the cells of diagnostic interest but also irrelevant cells in the same tissue sections. Some well-known examples include the expression of vimentin in stromal cells, the expression of keratin in a subset of dendritic reticulum cells in lymph nodes, or the expression of S100 protein in a rare lymphoid cell or macrophage in lymph nodes. The diagnostic significance of a positive staining reaction thus depends on correct identification of the cells of diagnostic interest by routine morphology. As pointed out by the authors, PAX-2 expression by lymphoid cells was described in detail in one of our previous publications,2 which is referenced in the article in discussion. The morphology of these cells, however, is such that they can be quite clearly identified as lymphoid cells, rather than metastatic neoplastic cells. These positively stained lymphoid cells are indeed clearly illustrated in Figure 4 in the article in discussion.1 This rather obvious diagnostic caveat is applicable for the use of PAX-2 as a marker for metastatic renal cell carcinoma. Nevertheless, this caveat should have been clearly mentioned and discussed as a diagnostic pitfall in the article in discussion. We are grateful that the authors help reiterate this well-known but important diagnostic caveat.

1.
Ozcan
A
,
Zhai
QJ
,
Javed
R
,
et al
.
PAX-2 is a helpful marker for diagnosing metastatic renal cell carcinoma: comparison with the renal cell carcinoma marker antigen and kidney-specific cadherin
.
Arch Pathol Lab Med
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2010
;
134
(
8
):
1121
1129
.
2.
Zhai
QJ
,
Ozcan
A
,
Hamilton
C
,
et al
.
PAX-2 expression in non-neoplastic, primary neoplastic, and metastatic neoplastic tissue: a comprehensive immunohistochemical study
.
Appl Immunohistochem Mol Morphol
.
2010
;
18
(
4
):
323
332
.

The authors have no relevant financial interest in the products or companies described in this article.