Abstract
Head activator (HA) is an undecapeptide (Fig. 1) identical in sequence from hydra to mammals (Bodenmuller and Schaller 1981; Schaller and Bodenmuller 1981). In hydra HA occurs as a gradient with maximal concentration in the head region. As a morphogen HA is required for head-specific growth and differentiation processes, hence its name (reviewed in Schaller 1996; Schaller et al. 1996). In mammals HA is a growth factor for early nerve and neuroendocrine development, and it has modulatory functions later in life (Kajiwara and Sato 1986; Schaller et al. 1989; Quach et al. 1992; Ulrich et al. 1996; Kayser et al. 1998).
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