keratin

ker·a·tin   (kěr'ə-tĭn)    n.  A tough, insoluble protein substance that is the chief structural constituent of hair, nails, horns, and hooves. [Greek keras, kerāt-, horn; see ker-1 in Indo-European roots + -in.] ke·rat'i·nous (kə-rāt'n-əs) adj.

keratin 

Main Entry: ker·a·tin
Pronunciation: 'ker-&t-&n
Function: noun
: any of various sulfur-containing fibrous proteins that form thechemical basis of horny epidermal tissues (as hair and nails) and are typically not digested by enzymes of the gastrointestinal tract —see PSEUDOKERATIN

keratin ker·a·tin (kěr'ə-tĭn)
n.
Any of a group of scleroproteins or albuminoids that contain large amounts of sulfur and are the chief structural constituents of hair, nails, and other horny tissues.