Such a decrease of the HPCs also occurs in RCC patients treated with sunitinib (Vroling et al, 2009) and might be related to the Flt-3 inhibitory effects of both TKI sorafenib and sunitinib, as Flt3-signalling is required for HPC proliferation and mobilisation into the circulation (Gabbianelli et al, 1995; Faivre et al, 2007; Kumar et al, 2009). Here, FLT3 is linked to renal cell carcinoma.