Lets look at definitions and applications of the methods in your question:

The western blot (also known as western blotting), is a widely used analytical technique in molecular biology and immunogenetics to detect specific proteins in a sample of tissue homogenate or extract (or to detect a specific protein in a blood sample for instance).

It is noteworthy to mention that ELISA is a more simple and faster procedure than Western blotting, which is less specific. Western Blotting is a highly successful testing method for confirming positive results from ELISA tests. It is also used as a confirmatory test as it is difficult to perform and requires a high skill level.

On the other hand, real-time polymerase chain reaction (PCR) is a technique of molecular biology based on the polymerase chain reaction. It monitors the amplification of a targeted DNA molecule during the procedure (in real time), not at its end, as is the case in the conventional PCR. Real-time polymerase chain reaction can be used quantitatively and semi-quantitatively.

Hello Ali Salehi

Western Blot is an antibody-based technique. If you must detect specific protein molecule in a complex sample such as tissue homogenates and cell lysates, you may do so by using Western Blot. You may use Western Blot when you must measure the amount of protein expression as well as when you have to evaluate the size of your protein of interest.

On the other hand, if you must study gene expression, real-time PCR may be used to investigate changes (either increase or decrease) in the expression of a particular gene or a set of genes by measuring the abundance of gene-specific transcript. For instance, you may monitor the response of a gene to a particular treatment with a compound or a drug. You may also try quantitative changes in expression levels or the overall pattern of expression.

Real-time PCR being the most sensitive technique, you could also use it for genotyping, to measure copy number variation of specific sequences in the genome, pathogen detection and measuring RNA interference.

Best.

I meant, for a molecule like MMP9, should I do method western blot or method real-time pcr?@M. R. Mozafari