Transfection efficiency refers to the proportion of cells that have successfully taken up and expressed foreign DNA following a transfection procedure. The efficiency is typically expressed as a percentage of the total number of cells in the sample that have been successfully transfected. A high transfection efficiency means that a large proportion of cells have taken up and expressed the foreign DNA, while a low transfection efficiency means that only a small proportion of cells have been successfully transfected. The transfection efficiency is an important factor to consider in transfection experiments, as it can impact the results of the study and the interpretation of the data. The transfection efficiency can be influenced by many factors, including the type of transfection reagent, the size and type of the foreign DNA, the cell type, and the conditions of the transfection procedure.