Relative frequencies of phenotypic LT-HSCs between wild-type and Id1^−/− marrow. Numbers of cells meeting the given LT-HSC phenotypes per 100 000 WBM cells were calculated. Bars show the average of at least 3 samples per phenotype. Error bars indicate SEM. Statistically significant differences between wild-type and Id1^−/− samples were determined by means of the Student t test (P values).