This paper reports the insertion of C causing frameshift and termination.
32 families with cerebrotendinous xanthomatosis were examined for mutations in CYP27. This variant was seen in four families, but in all cases this was cis with a 5-6insC mutation previously reported as pathogenic (Segev et al.). Presumably that insertion was causal, not the substitution predicted for this downstream location (which would not have been transcribed). This variant may have been ancestral to the pathogenic insertion variant.
This paper studies the structure of the protein and attempts to explain why particular reported variants have pathogenic impact. It explains why this variant may have had a pathogenic impact, but appears to be unaware that all reported instances of the variant were coupled with the upstream frameshift mutation.