The present invention includes methods for treating a proliferative disorder in a subject with mutated or constitutively active FLT3 in a subject relapsed/refractory to one or more prior tyrosine kinase inhibitors comprising: obtaining a tumor sample from the subject that is relapsed/refractory to one or more prior tyrosine kinase inhibitors; measuring expression of mutated or constitutively active FLT3 mutant in the tumor sample; and administering to the subject a therapeutically effective amount of crenolanib or a pharmaceutically acceptable salt thereof sufficient to treat the proliferative disorder.