Functions as an activator of the E3 ubiquitin protein ligase ITCH in the ubiquitination of the CXCL12-activated CXCR4 receptor. Thereby, triggers CXCR4 endocytosis and desensitization, negatively regulating the CXCL12/CXCR4 signaling pathway. . This is the function of ENSG00000135063 (ENTREP1, endosomal transmembrane epsin interactor 1).