Detection of β-gal⁺ cells in the blood of transgenic mice. FACS-gal analysis of viable peripheral blood leukocytes from (A) a (C57BL/6 × SJL) F1 negative control mouse (broken line) and a Rosa26 positive control (solid line) and (B) a HS21/5 vav-βgal transgenic mouse. Cells were analyzed for β-gal activity (represented on the X-axis by mean fluorescence units). The proportion of expressing cells was determined after setting the window to yield less than 5% β-gal⁺ for a negative control mouse run in parallel.