Identification of myonuclei and satellite cells from growing 3F-nlacZ-E and Myf5nlacZ/+mice. Myofibres were isolated from P14 mice, fixed and incubated in X-gal and DAPI. Myonuclei are revealed in 3F-nlacZ-E myofibres by β-galactosidase activity producing a blue reaction product (a), whereas satellite cells are visible as DAPI-positive nuclei, since the X-gal reaction product obscures the DAPI fluorescence in myonuclei (b). This enables clear discrimination between myonuclei and satellite cells, as seen in the merged image (c). By contrast, in Myf5nlacZ/+ mice, satellite cells are identified by their β-galactosidase activity on isolated myofibres (d), whereas myonuclei are DAPI-positive nuclei that do not contain the blue X-gal reaction product (e and f). Scale bar equals 50 μm.