colony forming units in spleen cfu s u2014 can be detected in vivo by forming macroscopic colonies in the spleens of lethally irradiated mice 8 12 days after the intravenous injection of hematopoietic cells till amp mcculloch 1961 for the standard cfu s proliferation assay the method of 3 h thymidine u201c suicide u201d is usually applied becker et al 1965 the method is based on incorporation of radiolabelled thymidine 3 h thymidine a precursor of dna into cells during dna synthesis the cfu s which are in s phase of the cycle at the time of testing are killed by the high radioactivity and therefore not able to form colonies in spleen thus the difference between the number of cfu s formed by the injection of the cell sample incubated without 3 h thymidine and the same cells incubated with 3 h thymidine shows the percentage of the proliferating cfu s in the original sample the inhibitor testing can not be done with the bone marrow stem cell population from unstimulated animals as far as the inhibitor only effects cycling cfu s which are as low as 7 10 of the total cfu s population in the bone marrow of normal.