Cysteine is one of 20 amino acids that are the building block of proteins used throughout the body. Protein forms the structure of all of the body's cells and tissues, including muscle tissue, internal organs, tendons, hair, skin and nails. Cysteine is a non-essential amino acid as it is one of 12 amino acids that can be produced naturally by the body. Acetyl L-Cysteine provides stability, increased bioavailability and is an altered form of the amino acid L-Cysteine. Cysteine is one of 20 amino acids that are the building block of proteins used throughout the body. Protein forms the structure of all of the body's cells and tissues, including muscle tissue, internal organs, tendons, hair, skin and nails. Cysteine is a non-essential amino acid as it is one of 12 amino acids that can be produced naturally by the body. Acetyl L-Cysteine provides stability, increased bioavailability and is an altered form of the amino acid L-Cysteine.