Biotin-X NTA (Biotin-X nitrilotriacetic acid, tripotassium salt) is used to detect histidine-tagged proteins (as little as 0.1 pmol) immobilized on nitrocellulose membranes. Biotin-X NTA can be removed from the histidine-tagged protein at pH 4.8, allowing the blot to be reanalyzed with another probe.