Reed-Sternberg Cells : PAX5
B-cell transcription factor PAX5 is positive in more than 90% of classic Hodgkin lymphomas (cHL). The nuclear expression of PAX5 in Reed-Sternberg (RS) cells is usually weaker (see larger cells in the left half of the image) than in reactive B cells (right half). Other transcription factors, including BOB1 and Oct2 are negative. If these two markers are strongly expressed, the diagnosis of B-cell lymphoma unclassifiable, with features between DLBCL and cHL should be considered. IRF4/MUM1 is strongly positive in the nuclei of RS cells. It is also positive in anaplastic large cell lymphoma.