The invention concerns novel DNA and amino acid sequences of monoclonal antibodies (mAbs) raised against lymphoblastoid cells and peptides to which the mAbs bind to. The invention also concerns diagnostic assays using said antibodies or peptides for detecting individuals with a high probability of having a malignant disease and, at times, for detecting an individual having a specific malignant disease. The invention further concerns pharmaceutical compositions comprising the mAbs or peptides of the invention for use in the treatment of various malignant diseases as well as methods for the treatment of malignant diseases using the mAbs or peptides of the invention.