A method and pharmaceutical compositions useful for inhibiting the growth of solid tumors (e.g., skin caner) are provided. Specifically, the method is effected by administering to a subject in need thereof an agent capable of upregulating the expression level and/or activity of at least a functional portion of Sef, wherein the functional portion being capable of inhibiting RTK-mediated cell proliferation and whereas upregulating can be effected by: expressing in cells of the subject an exogenous polynucleotide encoding at least a functional portion of Sef (e.g., SEQ ID NOs:4, 8 or 9); increasing expression of endogenous Sef in cells of the subject; increasing endogenous Sef activity in cells of the subject; and introducing an exogenous peptide and/or exogenous polypeptide including at least a functional portion of Sef to the subject. Also provided are methods and kits for diagnosing and staging of cancer (e.g., breast cancer, thyroid cancer, prostate cancer) by detecting the expression level of hSef in a tissue sample, wherein a decrease in hSef expression level is indicative of cancer.