The Mix1 homeobox-like (MIXL1) gene encodes a paired class homeobox transcription factor that is involved in embryogenesis. Previous studies have shown that the MIXL1 gene product is expressed in B- and T-cell progenitors of normal bone marrow and, in some cell lines derived from hematopoietic neoplasms. The status of MIXL1 expression and subcellular localization in human lymphomas is unknown. Using a highly specific antibody, we assessed for MIXL1 expression in lymphoma cell lines of B- and T-cell lineage by reverse transcriptase-polymerase chain reaction, Western blot analysis, and immunohistochemistry. We also assessed for MIXL1 expression using immunohistochemical methods in 193 lymphoid tumors, including 140 B-cell non-Hodgkin lymphomas (NHL), 36 T-cell NHL, and 17 Hodgkin lymphomas (HL). MIXL1 was detected predominantly in the nuclear fraction of all cell lines tested and was predominantly nuclear in primary tumor specimens. Based on the distribution of the staining results (histogram), a 50% cutoff was selected for high versus low MIXL1 expression. High MIXL1 expression was detected more frequently in Burkitt lymphoma and diffuse large B-cell lymphoma compared with other types of B-cell NHL (P < .0001, chi(2) test). Most cases of T-cell NHL and all cases of HL also highly expressed MIXL1. Most plasma cell myelomas were negative for MIXL1, but rare cases had low MIXL1 expression. MIXL1 expression significantly correlated with proliferation index (Ki-67) in B-cell NHL (P < .0001). The frequent and high expression of MIXL1 in aggressive B-cell NHL, T-cell NHL, and HL suggests that MIXL1 may be involved in lymphomagenesis.