Preprotachykinin-1, (abbreviated PPT-1, PPT-I, or PPT-A), is a precursor protein that in humans is encoded by the TAC1 gene. The protein has four isoforms—alpha-, beta-, gamma-, and delta-PPT—which can variably undergo post-translational modification to produce neurokinin A (formerly known as substance K) and substance P. Alpha- and delta-PPT can only be modified to substance P, whereas beta- and gamma-PPT can produce both substance P and neurokinin A. Neurokinin A can also be further modified to produce neuropeptide K (also known as neurokinin K) and neuropeptide gamma. These hormones are thought to function as neurotransmitters which interact with nerve receptors and smooth muscle cells. They are known to induce behavioral responses and function as vasodilators and secretagogues. Alternative splicing of exons 4 and/or 6 produces four known products of undetermined significance.