Guts has a number of meanings, both as a single word and as an acronym: It has been suggested that this article or section be merged with Backronym and Apronym (Discuss) Acronyms and initialisms are abbreviations, such as NATO, laser, and ABC, written as the initial letter or letters of words, and pronounced on the basis of this abbreviated written form. ...
Guts is a skill in the PlayStation games Star Ocean: The Second Story and Valkyrie Profile: Lenneth that allows a chance for a character to ignore damage that would otherwise make their hit points go into the negative (resulting in death).
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GUT theory specifically predicts relations among the fermion masses, such as between the electron and the down quark, the muon and the strange quark, and the tau lepton and the bottom quark for SU(5) and SO(10).
GUT models generically predict the existence of topological defects such as monopoles, cosmic strings, domain walls, and others.
Some GUT theories like SU(5) and SO(10) suffer from what is called the doublet-triplet problem in that these theories predict that for each electroweak Higgs doublet, there corresponds a colored Higgs triplet field with a very small mass (small, meaning many orders of magnitude smaller than the GUT scale here).