Mice with the nu mutation have been shown to have a reduced incidence of spleen colony-forming units (CFU-S), with a higher variability between individual mice compared to +/+ mice. The presence of these defects also in nu/+ mice indicates that this mutation is a codominant trait and that the hematopoietic changes may not be solely caused by thymus dysgenesis. The proportion of CFU-S synthesizing DNA (approximately 40%) has not been significantly different between +/+, nu/+, and nu/nu mice.