Almost a century ago Paul Ehrlich introduced the "magic bullet" concept of targeting therapeutic agents to specific tissues in order to reduce systemic toxicity. Due to the advances in hybridoma technology in the 1980s, monoclonal antibodies (MAbs) with their exquisite affinity to tumor antigens have become powerful tools in the treatment of cancer, especially when linked to therapeutic agents such as radionuclides, drugs, toxins, or enzymes. It can now be expected that such agents will lead to new cancer treatments with high therapeutic success rates. However, there are major problems in...