 He showed that cyclin expression is rate-limiting for oncogene-induced breast tumor growth "in vivo".He has been a pioneer of the non-canonical functions of cyclins and was the first to show that cyclins regulate diverse function including miRNA biogenesis, cellular migration, mitochondrial metabolism (the Warburg effect), angiogenesis and nuclear receptor function and hormone signaling "in vivo".Thirdly, Pestell defined key target genes required for breast cancer stem cell expansion in vivo including p21Cip1, c-Jun, the canonical NF-κB pathway, the cell fate determination pathway protein DACH1, and CCR5.