Astrocytic tumors are the most common malignancies of the central nervous system, accounting for more than 60% of all primary brain tumors. The prognosis for high grade astrocytoma patients is dismal and there is no curative treatment, today. A molecular hallmark of astrocytic tumors is dysregulated receptor tyrosine kinase signaling, especially of the epidermal growth factor receptor (EGFR, ErbB1). The aim of the present thesis was to identify endogenous human proteins that downregulate the function of the ErbB1 receptor. We identified a human gene family, the leucine-rich repeats and immunoglobulin-like domains family (LRIG), consisting of LRIG1, LRIG2 and LRIG3, which might fulfill this criterion. Two candidates were identified, LRIG1 and LRIG2, which genes were localized to regions frequently deleted in human cancers, chromosome bands 3p14 and 1p13, respectively. LRIG1 and LRIG2 mRNA were expressed in all tissues analyzed, with high expression in brain and other organs. The LRIG mRNA were predicted to encode integral membrane proteins. Antibodies against LRIG1 and LRIG2 were developed and the protein expression was analyzed. LRIG1 was found to have an apparent molecular weight of 143 kDa and was expressed in most tissues with high expression in glandular tissues of the breast and prostate, and the peptic cells of the stomach. LRIG2 was slightly smaller and had an apparent molecular weight of 132 kDa. The LRIG proteins were localized to the cell surface and to perinuclear structures, where LRIG1 co-localized with the trans-Golgi network and early endosomes. LRIG1 was found to restrict growth factor signaling by enhancing receptor ubiquitylation and degradation. We showed that LRIG1 interacted with all four members of the ErbB family and induced their downregulation. The interaction with ErbB1 involved both the LRR-domains and the Ig-like domains of LRIG1. LRIG1 enhanced receptor degradation by recruiting the E3 ubiquitin ligase c-Cbl to the LRIG1-ErbB1 complex. LRIG1, LRIG2, and LRIG3 were expressed in glioma cell lines and tumor tissues. The LRIG expression was analyzed in 404 astrocytic tumor samples. We found that perinuclear LRIG protein expression correlated with increased survival of patients with astrocytic tumors. Especially perinuclear LRIG3 showed strong correlations with patient survival and tumor cell proliferation index. In summary, this thesis contains the discovery and characterization of human LRIG1 and LRIG2. LRIG1 was found to interact with ErbB receptors and downregulate their function. In a clinical material, expression of LRIG proteins correlated with survival in patients with astrocytic tumors.