Colony assay led to the identification of the colony-stimulating factors beginning in the late 1960s. “Feeder” layers, containing various types of white blood cells in a semisolid medium, were placed in a small laboratory dish (1). Bone marrow cells (including stem cells) were added to form a second layer (2). When the dish was incubated, colonies of white blood cells formed in the second layer (3). The colonies were counted and the cells identified (4). When the contents of the first layer were varied, different types of colonies formed, implying the existence of a range of colony-stimulating factors. Illustration by Kenneth Probst. Adapted with permission from original artist Patricia J. Wynne. Golde DW, Gasson JC. Hormones that stimulate the growth of blood cells. Sci Am. 1998;259:67.²⁵