One of these cells types, characterized by both the expression of the Venus transgene and the ES cells marker SSEA - 1 (V+S +), appears to represent an early step in primitive endoderm specification.
One model, the «specification model», postulates that Toddler's primary role is to promote the specification of endoderm, which when defective leads to abnormal migration of mesendodermal cells (Chng et al., 2013).