Acute Myeloid Leukemia (AML) is an aggressive cancer with poor overall survival due to frequent relapse. FMS-like tyrosine kinase (FLT3) is the most commonly mutated genes in AML, and leads to higher relapse rates. Several FLT3 inhibitors have been developed, including gilteritinib. Our lab developed a two-step model of early and late resistance to study the mechanism of resistance and relapse to FLT3 inhibitors. Early resistant AML cells are dependent upon the marrow microenvironment for survival. Over time, intrinsic resistance allows independent growth and results in late resistance, leading to disease relapse.