Tissue-specific cancers originate in organ-or tissue-specific cells with the potential to proliferate. Recent reports have shown that every adult tissue may have its own stem cells. Cancer cells and organ-and tissue-specific stem cells and other progenitor cells are strikingly similar: both types self-renew, proliferate indefinitely, migrate, differentiate and express telomerase. Telomerase is a ribonucleoprotein that adds on nucleotides to the telomeres of chromosomes and so protects them from degradation, recombination and fusion at double-stranded DNA breaks. Cancer cells differ from normal tissue-specific stem cells only in their uncontrolled growth and altered genotypes. Based on the many similarities between cancer cells and stem cells, I propose that cancers (both solid and hematopoietic) arise from tissue-specific stem cells. Some such stem cells are always present among the stem cells in the peripheral blood of cancer patients and some of their asymptomatic first-degree relatives. It may therefore be possible to diagnose cancer predisposition early by identifying these circulating stem cells by their genetic defects. I, therefore, suggest that drug development for cancer treatment be directed towards tissue-specific stem cells.