In the 1960s, scientists could readily identify several types of bone marrow blood cells under the microscope. But Becker, Till & McCulloch were looking for the elusive blood stem cell. Hypothetically, a stem cell would give rise to all the other cells but no research lab had any evidence to demonstrate its existence. 

Step 1 of their experiment was to apply radiation to a bone marrow cell sample. The radiation disrupts the chromosomes in the cell nucleus, creating a kind of unique identification tag for each cell.

Step 2 was to grow colonies from the irradiated cells. After 11 days they saw colonies containing multiple cell types.  And the cells within a colony all had the identical chromosomal marking, demonstrating that they must be descendants of an original cell. 

This was the first positive evidence that a single cell could indeed give rise to all the different types of blood cells. 

From this experiment, they were able to establish two key properties about stem cells.

1. Stem cell can proliferate to become more stem cells and
2. Stem cells can differentiate to become many different types of cells.