I would like to measure transcription factors ( e.g FoxP3) and phospho proteins (e.g. pSTAT5) in the same sample. Which buffer should I use?

We recommend that you use the most stringent buffer system, which is in this case, the Cell Signaling Buffer Set A. Be careful if you are also performing a surface staining based on PE or APC conjugates, as this must be done after the methanol step.