Tagraxofusp is a truncated human IL3-diphtheria toxin fusion protein approved by the FDA in December 2018 after promising phase I studies that subsequently led to the largest multicenter study in patients with BPDCN conducted by Pemmaraju et al. in 47 people over four years, excluding patients with CNS involvement and those with albumin less than 3.2 g/dL due to the risk of capillary leak syndrome. Here, IL3 is linked to capillary leak syndrome.