To solve this problem, we need to identify which option is not an application of PCR (Polymerase Chain Reaction). Let's analyze each option:$\newline \bullet$Option 1: Molecular diagnosis$\newline$ - PCR is widely used in molecular diagnosis to detect the presence of pathogens or genetic disorders by amplifying specific DNA sequences.$\newline$ - Therefore, this is an application of PCR.$\newline \bullet$Option 2: Gene amplification$\newline$ - PCR is specifically designed for gene amplification, allowing for the exponential increase of a specific DNA segment.$\newline$ - Therefore, this is an application of PCR.$\newline \bullet$Option 3: Purification of isolated protein$\newline$ - PCR is a technique used to amplify DNA, not for purifying proteins.$\newline$ - Therefore, this is not an application of PCR.$\newline \bullet$Option 4: Detection of gene mutation$\newline$ - PCR can be used to detect gene mutations by amplifying and then sequencing or analyzing the DNA for specific mutations.$\newline$ - Therefore, this is an application of PCR.$\newline$Based on the analysis, the correct option is Option 3: Purification of isolated protein, which is not an application of PCR.