Effect of neomycin on GM colony growth from ROSA 26 (□) and B6/129 F2 (▪) marrow. GM colonies were grown in the presence of increasing concentrations of G418, a neomycin analogue. The number of colonies that grew in the presence of G418 was compared with the number of colonies that grew in media alone and expressed as a percentage. ROSA 26 GM colonies grow at extremely high concentrations of G418. In contrast, GM colonies from B6/129 F2 marrow grow only at G418 concentrations less than 1,000 μg/mL. Concentrations given are of total drug. The specific activity of the lot of G418 used in all experiments was 705 μg/mg.