I am working on a genetics assignment and am completely stumped on this problem. I have looked at it for hours and cannot figure it out. The biggest problem I am running into what kind of mutagen could induce these specific changes. I've tried various ways of frame shifting, but nothing seems to be working here. Please help!

Here is the problem:

An experimenter is studying a gene that codes for the polypeptide phe-his-gly-cys-gly

He treats the cell with different mutagens that cause the following changes to the protein:

Treatment 1: phe-his-gly-cys
Treatment 2: ser-his-gly-cys-gly

For each treatment, describe the exact mutagenic event (e.g. transition, transversion, frameshift, etc), the most likely codon sequence, and the type of mutagen used.