CD44 and keratoconus: We then purified CD44+, CD133+ or CD44+CD133+ (and corresponding marker-negative) HPCa cells from 4 GS6 tumors (HPCa 9, 10, 13, and 16), 4 GS7 tumors (HPCa4, 6, 8, and 12), and 1 GS9 tumor (HPCa42) and implanted increasing numbers of cells (from 1,000 to 2 million) subcutaneously (for the GS9 HPCa cells) or in the KC or AP (for the rest) of the male NOD/SCID mice and we observed 0/225 outgrowths (Table 6 in ref. 14).