Clusters of islet-1-positive cells in 129SvJ and 129SvJ × C57BL/6J mouse hearts. Both day 1 postnatal 129SvJ (b, d) and 129SvJ × C57BL/6J (a, c) hearts contained clusters of the first type (a, b) and the second type (c, d). Scale bars: (a, c) 50 μm; (b, d) 5 μm. Color images available online at www.liebertonline.com/scd