The transgenic construct contains mouse Runx1 enhancer element sequence (eR1) and the mouse Hspa1a (heat-shock protein 68) basal promoter sequence driving the expression of cre/ERT2. More than 10 founder lines were generated. The line TG(5-2) with higher copy number and TG(3-7) with lower copy number used for studies. Although the former had higher Cre recombinase activity than the latter, the results obtained using these lines were qualitatively indistinguishable.