ERBB4 and neoplasm: Previously, TrkA-Trk-A, HGFR, Erb-B4 have been shown increased in TICs in a variety of tumors, however, these protein levels were not different between non-TICs and TICs enriched by hypoxia and serum depletion, suggesting there are more than one subpopulations of TICs in tumor, each can be enriched by different conditions.