Calculated 2° CFC expansion ratios as a function of assay plating densities. Fresh and expanded BM MNCs were assayed at different plating densities (the data set represented by triangles in Fig 3) and the 2° CFC frequencies at each density were used to calculate an expansion ratio. Calculated 2° CFC expansion varied significantly as the different points of Fig 3 for fresh and expanded 2° CFC frequencies were used for the calculation.