The early development of the heart can be viewed as a series of steps in which cardiomyocytes are parsed into distinct classes (Harvey, 2002). The first of these is the separation of cardiac precursors into primary and secondary heart fields (PHF and SHF, respectively). This has been demonstrated by a number of complementary lines of research using gene expression, transgenic reporters, and lineage tracing (Kelly et al., 2001; Mjaatvedt et al., 2001; Waldo et al., 2001; Kelly and Buckingham, 2002; Cai et al., 2003). The timing of this separation was determined by an elegant retrospective clonal analysis showing that the pre-septation heart is comprised of two populations of cells that become clonally distinct shortly after gastrulation (Meilhac et al., 2004). The physical locations of these two populations prior to forma-