Fig. 1. Schematic pathway for the determination of LR asymmetry. LR asymmetry proceeds from a poorly understood initial process that orients direction (1) to propagation and amplification by cascades of gene expression that culminate in production of a Nodal protein on the left side of the embryo (2). The range of Nodal action is constrained by lefty proteins, in particular at the dorsal midline. A principal function of Nodal proteins is to regulate expression of Pitx2c and other proteins that influence morphogenesis in asymmetrically developing organs (3). Modified figure reproduced, with permission, from the Nature Publishing Group (http://www.nature.com) (Hamada et al., 2002).

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