The invention relates to a diagnostic method and related compositions for characterizing disseminated and micrometastasized cancer cells in body fluids using DNA, mRNA and/or protein. According to the method, DNAs, mRNAs and/or proteins of cancerous and non-cancerous cells obtained from a body fluid (i.e., cell preparations that are not enriched for cancer cells) and DNAs, mRNAs and/or proteins of cancerous cells that are removed from the body fluid (i.e., cell preparations that are enriched for cancer cells) are analyzed, to determine the presence or expression in body fluid cells of at least one cancer-specific gene that is not expressed in non-cancer cells or of at least one cancer-associated gene, and of at least one cancer-specific or cancer-associated gene in cancer cells removed from the body fluid cells (i.e., in isolated cancer cells). Also provided are compositions and methods for testing antineoplastic substances and anticancer therapies by characterizing a cancer-specific or a cancer-associated gene, and examples of cancer-specific and cancer-associated genes, including tissue-specific genes, tumor suppressor genes, oncogenes and others.