The ** G-factor** provides a measure of how "

The standards of "**normality**" used here have been derived from an
analysis of **163** non-homologous, high-resolution protein chains
chosen from structures solved by **X-ray crystallography** to a
resolution of **2.0Å** or better and an ** R-factor**
no greater than

The analyses provided the **observed** distributions of **phi-psi**,
**chi1-chi2** and **chi1** values for each of the **20** amino
acid types. These distributions were then divided into **cells**. For
example, each residue type's **Ramachandran plot** of **phi-psi**
values was divided into **45** x **45** cells. The numbers of
observations in each cell were used to calculate the **probability** of
a given residue type having a given **phi-psi** combination. The
probabilities were, in turn, used to compute a **log-odds score** for each
cell. Log-odds scores can be summed, rather than multiplied
like probabilities; therefore, taking meaningful averages becomes possible.

When applied to a given residue, a **low** *G*-factor indicates
that the property corresponds to a low-probability conformation. So, for
example, residues falling in the disallowed regions of the **Ramachandran
plot** will have a **low** (or very negative)
*G*-factor. Similarly for unfavourable **chi1-chi2** and **chi1**
values.

Thus, if a protein has many residues with low *G*-factors it
suggests that something may be amiss with its overall geometry.