The early embryo is initially bilaterally symmetrical. One of the first distinct indications of asymmetry in the embryo occurs during heart looping. The midline tubular heart begins to bend to the right to form a C-shaped structure around 30 hr of development in the avian model. A molecular basis for heart asymmetry and direction of looping is not known, although factors inherent to the myocardium are believed to underlie looping. A left-right asymmetric localization of a specific molecule in the bilateral heart forming regions has not been reported previously. One molecule that we are calling flectin (flectere, in L., to bend or to loop) shows a bilateral asymmetric localization early in the heart forming mesoderm and continues to be expressed asymmetrically in a highly organized manner in the cardiac jelly during heart looping. This large extracellular matrix molecule has been identified using a monoclonal antibody F-22 (Mieziewska et al., 1994a,b). Flectin shows a discrete spatiotemporal pattern of extracellular matrix expression during avian heart development. An asymmetric expression of flectin is observed during heart development at stage 7+/8- (approximately at 24 hr of development around the 3-somite stage). It is predominantly expressed in the left precardiac mesoderm at this developmental period. Between stages 12 and 14, flectin continues to be asymmetrically expressed in the myocardium and is localized at high levels on the basal side of the myocardium and within the cardiac jelly extending to the endocardial cell surfaces. In the same plane of the looping part of the heart it is differentially organized within the cardiac jelly on the convex side and in the outer loop areas. A reduced expression is apparent anteriorly and posteriorly along the tubular heart. The initial asymmetry of localization is maintained throughout the tubular heart. At stage 22 (Embryonic Day 3.5), intensity of immunolocalization of flectin is significantly decreased, with left-right asymmetry becoming less discernible or absent. It again is expressed in Day 10 embryonic hearts. Flectin expression appears to be modulated by retinoids. In vitamin A-deficient quail embryonic hearts that do not loop (Dersch and Zile, 1993; Twal et al., 1995), flectin protein expression is decreased and disorganized, as are other extracellular matrix components comprising the cardiac jelly.