Makes a pen made from casein. (milk protein) Those who are severly allergic to milk, and especially those who have a habit of chewing on your pens may want to avoid these pens.
From their web site: One Hundred Casein
"Casein is a rare and natural material derived from milk by-products. It takes 12 months to manufacture and cure casein to a product suitable for pen manufacture which imparts the highest of lustres. Available in fountain pen (with the full variety of nib widths) and ball point in very limited numbers."