In mouse model generation, obtaining positive cell clones that stably integrate and express a transgene is a crucial step. Among selection methods, neomycin resistance (NeoR) and puromycin resistance (PuroR) are the most commonly used.

The core principle involves co-transfecting the gene of interest with a resistance gene into host cells (e.g., embryonic stem cells). The cells are then cultured in medium containing the corresponding antibiotic. The sustained antibiotic pressure eliminates cells that have not stably integrated and functionally expressed the resistance gene, ultimately enriching stably transfected positive clones.
 

Neomycin Resistance (NeoR) Selection
 

Neomycin resistance selection relies on the aminoglycoside antibiotic G-418 (Geneticin). G-418 works primarily by irreversibly binding to ribosomes, inhibiting protein elongation, thereby blocking protein synthesis. It is broadly toxic to both prokaryotic and eukaryotic cells. The neoRgene (derived from transposon Tn5) encodes aminoglycoside phosphotransferase (APH(3')II). This enzyme covalently modifies G-418, inhibiting the antibiotic-ribosome interaction, thus inactivating the antibiotic and conferring resistance to the cell.
 

Characteristics:​ Its action is relatively slow, with a selection period typically requiring 10 to 14 days. It is the classic choice for selecting embryonic stem (ES) cells in the generation of genetically engineered mice.
 

Concentration:​ The effective concentration range is broad (200-1000 µg/mL). Because sensitivity to G-418 varies greatly between different cell lines, a "kill curve" experiment to determine the optimal selection concentration is essential before screening.
 

Experimental Protocol:
 

1.Determine Optimal Selection Concentration (Kill Curve Experiment):
 

Plate untransfected cells in the logarithmic growth phase at approximately 30% density in a 24-well plate.
 

Set up a G-418 concentration gradient (e.g., 0, 200, 400, 600, 800, 1000 µg/mL).
 

Replace the medium with fresh complete medium containing the corresponding G-418 concentration every 2-3 days.
 

Observe under the microscope after 7-10 days of culture. The lowest concentration that causes the death of all cells within this timeframe is the optimal selection concentration for that cell line. (Typically 400–800 µg/mL)
 

2.Stable Transfectant Selection:
 

24-48 hours after transfection, passage the cells at a low density (e.g., 1:10 to 1:20) into medium containing the optimal G-418 concentration.
 

Replace the drug-containing medium every 2-3 days to maintain selection pressure.
 

Continue selection for 10-14 days until cells in the negative control well are completely dead, and distinct resistant clones are visible.
 

Pick single clones using cloning cylinders or by limiting dilution, expand the culture for subsequent validation.
 

Puromycin Resistance (PuroR) Selection
 

Puromycin resistance selection utilizes the antibiotic puromycin. Puromycin is an aminonucleoside antibiotic whose chemical structure mimics the 3' end of a tRNA molecule. It can bind to an amino acid, substitute for an aminoacylated tRNA, and bind to the A site of the ribosome, incorporating into the growing peptide chain. Its mechanism of action involves mimicking the 3' end of aminoacylated tRNA. During translation, it incorporates into the nascent peptide chain, causing premature termination, thereby inhibiting protein synthesis. The puroRgene (i.e., the pacgene) encodes puromycin N-acetyltransferase (PAC), which inactivates puromycin through acetylation, conferring cell resistance.
 

 

Characteristics:​ It acts rapidly, effectively killing non-resistant cells within 24-72 hours. The selection cycle is relatively short, typically 5-7 days, making it particularly suitable for experiments requiring rapid enrichment of successfully transfected cells.

Concentration:​ The effective concentration range is lower (1-10 µg/mL). Puromycin is metabolically degraded by cells in the medium, usually requiring daily or every-other-day medium changes to maintain effective pressure.
 

Experimental Protocol:
 

1.Determine Optimal Selection Concentration:
 

Plate untransfected cells at about 20-30% density in a 96-well plate.
 

Set up a puromycin concentration gradient (e.g., 0, 1, 2, 4, 6, 8, 10 µg/mL).
 

Observe after 48-72 hours of culture to determine the lowest concentration that completely kills the cells. (Typically 2~5 µg/mL)
 

2.Stable Transfectant Selection:
 

24 hours after transfection, replace the medium with medium containing the optimal puromycin concentration.
 

Observe daily. Negative control cells should undergo massive death within 72 hours.
 

After 5-7 days of selection, surviving cell colonies are positive clones. They can be cultured as a mixed population or isolated as single clones based on experimental needs.
 

In practice, both systems have distinct characteristics. Neomycin resistance selection is a gentler process, suitable for drug-sensitive cell lines (like ES cells), and is the classic choice for establishing stable cell lines and mouse models. Puromycin resistance selection offers the advantage of speed and high efficiency. The choice of resistance selection system should comprehensively consider the cell type, the resistance gene carried by the expression vector, and the experimental timeline. Regardless of the choice, performing a preliminary experiment to determine the optimal selection concentration for the specific cell line is a critical step to ensure success.