Stimulation begins with contact between 5-HT and either a transporter or receptor, which may lead to activation of Ras or Rac or both. The signal then activates a NAD(P)H oxidase that has not been fully characterized, leading to release of reactive oxygen species (ROS) and the activation of the ERK1 or ERK2 MAP kinases. The consequent changes in gene expression culminate in both cellular hyperplasia and hypertrophy.