left-right determination factor 2 (Lefty2), also known as Left-right determination factor A (Lefty A), endometrial bleeding associated factor (left-right determination, factor A transforming growth factor beta superfamily), and Transforming growth factor beta-4, is a member of the TGF-beta family of proteins. This protein is secreted and plays a role in left-right asymmetry determination of organ systems during development. Lefty2/Lefty A may also play a role in endometrial bleeding. Mutations in Lefty2/Lefty A have been associated with left-right axis malformations, particularly in the heart and lungs. Some types of infertility have been associated with dysregulated expression of this gene in the endometrium. Alternative processing of this protein can yield three different products. Depletion of both Lefty1 and Lefty2 causes unchecked Nodal signaling, expansion of mesendoderm, and loss of ectoderm. The expansion of mesendoderm correlates with an extended period of rapid cellular internalization and a failure of deep-cell epiboly. The gastrulation defects of embryos depleted of Lefty1 and Lefty2 result from the deregulation of Squint signaling.