A method of radiation-free hematopoietic stem cell (HSC) transplantation comprises administering to a mammalian subject one or two doses of 2 to 10 mg/kg body weight of a purine base analog, such as 6TG as a pre-conditioning step. The method further comprises engrafting into the subject hypoxanthine-guanine phosphoribosyltransferase (HPRT)-deficient donor HSCs within 48 to 72 hours of the pre-conditioning step; and administering to the subject about 1 to 5 mg/kg of the purine base analog every two to four days for two to eight weeks following the engrafting step. The method is performed in the absence of pre-conditioning via radiation. The subject is therefore not treated with myeloablative radiation in preparation for transplantation, and thus the subject is free of myeloablative radiation-induced toxicity.