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Blue-White Select Screening Reagent

for selection of recombinant bacterial clones


Quality Level


for molecular biology




sufficient for 125 dishes (90 mm)

shipped in

dry ice

storage temp.


General description

Sigma′s Blue-White Select Screening Reagent is a chromogenic substrate for β-galactosidase, used to determine the presence or absence of a cloned DNA insert in bacteria growing on agar plates. Blue-White Select Screening Reagent is designed for blue-white selection of recombinant bacterial colonies with the lac+ phenotype.

Features and Benefits

  • Intense color contrast for easy colony selection
  • Convenient, ready-to-use solution


40 mg/mL IPTG
40 mg/mL X-Gal
(prepared in DMSO)


When Blue-White Select Screening Reagent is cleaved by β-galactosidase, the resulting product will form a blue precipitate. Lac+ colonies grown in the presence of Blue-White Select Screening Reagent remain white in color, allowing for easy differentiation between lac+ and lac- colonies.

The technique is based on vectors such as the pUC and the M13mp series that carry a fragment of the β-galactosidase gene encoding an a-fragment of β-galactosidase. Exploitation of these vectors requires use of a bacteria strain carrying the complementing gene fragment to allow the assembly of an active complex, resulting in the formation of blue colonies. Disruption of the ß-galactosidase gene by insertion of a DNA fragment into the vector′s multiple cloning site results in the loss of functional β-galactosidase activity; these colonies remain white, allowing for easy differentiation between lac+ and lac- colonies.

Legal Information

Blue-White Select is a trademark of Sigma-Aldrich Co. LLC

Storage Class Code

10 - Combustible liquids



Flash Point(F)

188.6 °F

Flash Point(C)

87 °C

Personal Protective Equipment

dust mask type N95 (US), Eyeshields, Gloves

Certificate of Analysis

Enter Lot Number to search for Certificate of Analysis (COA).

Certificate of Origin

Enter Lot Number to search for Certificate of Origin (COO).


Blue-White Select™ Screening Reagent

Blue/white color selection is a routine technique employed by molecular biologists. This technique simplifies the differentiation between colonies/plaques that contain a cloning vector without an insert and those that contain a vector harboring an insert of interest.

Our team of scientists has experience in all areas of research including Life Science, Material Science, Chemical Synthesis, Chromatography, Analytical and many others.

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