Prothymosin alpha (ProTalpha) is an abundant highly acidic protein found in the nuclei of virtually all mammalian cells. The expression of this protein is increased in proliferating mammalian cells. However, the function of this molecule is still controversial. Here I present a model explaining the role of this protein in chromatin decondensation through its interaction with histone H1. beta-thymosins are a family of small actin-binding peptides widely distributed in eukaryotic cells. Here I will focus on thymosin beta-4, the most abundant member of this family. In particular, I will discuss its expression in the mammalian development of cardiovascular and nervous systems as well as its implications in neuronal plasticity.