The beta thymosins are a family of related peptides, initially isolated from calf thymus but known to be present in a wide variety of mammalian and other vertebrate cells and tissues. Thymosin beta 4 was the first member of the family to be characterized. Although TMSB4 was initially proposed to be a thymic hormone acting at early stages of T cell maturation, the high concentration of the protein and its mRNA in a number of other tissues and cells, as well as the lack of an identifiable secretory signal sequence, suggested that it had a general function in many cell types. Thymosin beta 10 is closely related in sequence to TMSB4 and is also an actin sequestering protein.