How would you prove the mouse was immortal???
If you take the lifespan of a mouse at say 2 years, to make it live to 4 would not prove immortality.
Or, if you take immortality as it would not die, then the fact you can still kill it shows that it is not immortal.
Plus, how would anyone know if a man was immortal or not? No one would be around long enough to tell.
Unless of course you went down the road of a mouse that did not age.
I am not just being pedantic here I promise But you have to explore what you are proving first before you prove it.
To answer your question though, I'd say the basics would be you have to show it was both replicable and repeatable and to show it in other genetically homologous organisms too.