Complex sentence: IL-1α is a unique member in the cytokine family in the sence that the structure of it's initially synthesized precursor molecule does not contain a signal peptide (same is known for IL-1β and IL-18).
Split into simpler sentences: IL-1α is a unique member in the cytokine family. It is initially synthesized as a precursor molecule without a signal peptide (same is known for IL-1β and IL-18).