Normal mouse blood-forming tissue contains a class of cells which, on being transplanted into heavily irradiated mice, can proliferate and form macroscopic colonies in the spleen. Often, within a given colony, the cells which are observed indicate that differentiation is occurring along three lines, into cells of the erythrocytic, granulocytic and megakaryocytic series, respectively. Unique radiation-induced marker chromosomes were used to obtain direct cytological evidence that the spleen colonies are clones. Thus, the advantages inherent in the clonal approach to studies of the genetic and physiological properties of cells are available for investigations on h歡topoiesis. Reproduced from Nature 1963(2 Feb); 197(4866): 452-454. The Nature website is www.nature.com