In normal cells, HER3 and HER4 are involved in numerous cellular processes. HER3 is unique because it lacks intrinsic tyrosine kinase activity, meaning it must pair with other HER family members to initiate downstream signaling. HER4, on the other hand, possesses kinase activity and can independently activate signaling pathways. Both receptors can activate the PI3K/AKT and MAPK pathways, which are critical for cell survival and proliferation.
