Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty ($1933$-$44$) the genetic material was thought to be a protein.

They worked to determine the biochemical nature of 'transforming principle' in Griffith's experiment.

They purified biochemicals (proteins, $DNA$, $RNA$ etc.) from the heat-killed $S$ cells to see which ones could transform live $\mathrm{R}$ cells into $\mathrm{S}$ cells.

The discovered that $DNA$ alone from $S$ becteria caused $\mathrm{R}$ bacteria to become transformed.

They also discovered that protein digesting enzymes (proteases) and $RNA$ – digesting enzymes [$RNases$] did not affect transformation, so the transforming substance was not a protein or $RNA$.

Digestion with $DNase$ did inhibit transformation suggesting that the $DNA$ caused the transformation.

They concluded that $DNA$ is the hereditary material, but not all biologists were convinced.