Peptidases or proteinases like proteinase K, pepsin or trypsin are used in ISH to digest proteins contained in the sample that might mask the targeted DNA/RNA. Clearing proteins generally helps with probe penetration and peptidase/proteinase treatment will always improve signal. However, it is not always necessary, as signal strength might already be excellent without, and in some cases might be detrimental (i.e. to preserve tissue integrity), especially if co-immunostaining is to be combined with ISH.