Transdifferentiation of B cell lymphoma of germinal center cell origin to histiocytic sarcoma has recently been described but is a rare occurrence. The cause for loss of B cell differentiation in these lymphomas is unknown. We investigated whether somatic hypermutation of the PAX-5 gene, a transcription factor that is important for maintaining B cell identity and is frequently mutated in B cell lymphomas of germinal center cell origin, might be a cause for loss of PAX-5 expression and thus B cell phenotype. However, no somatic hypermutation of the PAX-5 gene was detected in the two cases we studied. The molecular basis for transdifferentiation of B cell lymphoma to histiocytic sarcoma remains therefore unresolved.