The question asks to identify the methods used for studying protein-protein interactions from the given options.

Analyzing Interaction Techniques

Let's examine each technique listed:

• P. DNA footprinting: This technique is used to identify DNA sequences that are bound by proteins. It primarily studies protein-DNA interactions, not interactions between two proteins.
• Q. Yeast two-hybrid system: This is a widely used molecular biology tool designed specifically to detect protein-protein interactions in vivo. It relies on the reconstitution of a transcription factor upon interaction of the tested proteins.
• R. Ligase chain reaction (LCR): LCR is a method used for amplifying nucleic acid sequences. It is not applicable for studying protein interactions.
• S. Mass spectrometry: Advanced mass spectrometry techniques, often coupled with methods like immunoprecipitation, are powerful tools for identifying and validating protein-protein interactions within cells or complex mixtures.

Identifying Correct Methods

Based on the analysis:

• Technique Q (Yeast two-hybrid system) is a direct method for studying protein-protein interactions.
• Technique S (Mass spectrometry) is also a key method used to identify protein-protein interactions.
• Techniques P and R are not suitable for studying protein-protein interactions.

Therefore, the correct methods for studying protein-protein interactions are Q and S.