Stem cell biology has come of age. Unequivocal proof that stem cells exist
in the haematopoietic system has given way to the prospective isolation of
several tissue-specific stem and progenitor cells, the initial delineation
of their properties and expressed genetic programmes, and the beginnings of
their utility in regenerative medicine. Perhaps the most important and use
ful property of stem cells is that of self-renewal. Through this property,
striking parallels can be found between stem cells and cancer cells: tumour
s may often originate from the transformation of normal stem cells, similar
signalling pathways may regulate self-renewal in stem cells and cancer cel
ls, and cancer cells may include 'cancer stem cells'-rare cells with indefi
nite potential for self-renewal that drive tumorigenesis.