CD79a (MB1) is a B-cell marker with a wider range of positivity in the B-cell development spectrum compared to CD20 (mature B-cell phenotype).  CD79a may be expressed on malignant precursor B-cells and terminally differentiated B-cells (PAX-5 is not expressed in terminally differentiated B-cells).  Please note that in the setting of ALL, CD79a is not lineage specific, and up to 50% of T-ALL cases express CD79a.  AML cases with t(8;21) may also show CD79a expression (along with CD19, CD20, and TdT).