While the in vitro mammalian cell gene mutation test suggested that paraxanthine (up to 300 μg/ml) was mutagenic at the TK locus of cultured mouse lymphoma cells in the presence of metabolic activation, the in vivo mammalian bone marrow chromosomal aberration test demonstrated a lack of chromosomal aberration and toxicity to the bone marrow of Sprague Dawley rats (doses up to 100 mg/kg bw). The gene discussed is TKT; the disease is lymphoma.