Vincristine 2 mg will be administered to the participants intravenously over 5 minutes. Blood samples will be collected from an intravenous line inserted into the contralateral limb to that where the vincristine was given, at time zero (pre-vincristine treatment), immediately after vincristine administration (within 2-10 minutes upon completion of administration) and at 1, 2, 4 and 6 hours post-vincristine treatment. Patients will then at a later date receive chemotherapy treatment as prescribed by their primary oncologist.