Western blotting is a powerful technique to characterize a multitude of cellular proteins. The experimental protocol involves the comparison of levels of a given protein or its modifications between different cell preparations. To ensure quality during the analyses, it is necessary to probe for a protein known as “housekeeping gene product”. Below, we can see western blotting experiments after NIH3T3 fibroblasts were grown in different confluences, as indicated in the figure. RIPA extracts from the cells were loaded at 30 μg and resolved by gel electrophoresis. The blots were probed for alfa-tubulin, GAPDH, and Hsp90, as indicated. Numbers on the left refer to molecular weight markers. The experiments were conducted to point out the most suitable housekeeping gene product.

In this context, what is the most suitable housekeeping gene product and why?