A definitive proof of adaptive response belonging to the group of non-targeted effects came from a study by Iyer and Lehnert, demonstrating that non-irradiated human lung fibroblast cells (HFL-1) were able to adapt if grown in a medium transferred from HFL-1 cells, irradiated with either
0.1 Gy g-rays or 0.1 Gy a-particles (Iyer & Lehnert, 2002). The adaptation was shown as increased clonogenic survival after a challenging dose of 2 or 4 Gy g-rays, or 1.0 or 1.9 Gy a-particles, respectively. Adaptive response was found to be associated with a decreased level of p53 protein, increased level of intracellular ROS as well as increased level of DNA repair protein AP-endonuclease. The study suggested that non-targeted
cells are able to adapt after receiving an extracellular signal.