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Fig. 1. —Drawing shows asymmetric gene expression in cardiac looping. Paracrine signaling molecules (A) are secreted by cells into their surroundings, affecting neighboring cells. Transcription factors bind to DNA (B) and regulate transcription of DNA sequences (genes) into messenger RNA; note helix-loop-helix motif. Extracellular matrix proteins (C) are macromolecules that serve as support structure of cells.