SP4 and breast cancer: Knockdown of NR4A1, NR4A2 or Sp1/4 also decrease expression of NR4A1/Sp-regulated genes such as PD-L1, β1- and other integrins in breast cancer cells [20, 40, 42] and similar results were observed for CD71 in this study demonstrating that NR4A1 and NR4A2 in combination with Sp1 and Sp4 regulate basal expression of this key ferroptotic gene.