Amino acid is a building block to make a protein. Imagine a single amino acid as a bead, and a protein has many beads strung together. Cysteine is one of those “beads”, use to make a part of a protein, called the polymer. Imagine you took a bunch of strings with beads and tied them together to make a large necklace, thats the entire protein. In this case that protein is keratin. Cysteine is also found in bird feathers and horse hooves

Loool I learned this fun fact from reading ingredient labels! I actually found out that human hair contains L-cysteine. It is also found in bird feathers and horse hooves. L-cysteine is a dough conditioner that can be used to make breads softer. Some breads use asorbic acid but others use cysteine. One day I looked up cysteine and learned what you posted lol