It may be a form of Ilex crenata such as 'Sky Pencil', but we need a close up of the leaf arrangement to see if it's alternate or opposite.

Similar appearance to some of the larger-leafed euonymus shrubs that often have sporadic areas of yellow leaves. Prune them out unless you like the appearance.

One of the fastigiate Buxus cv

Leaves appear to be opposite, so Buxus rather than Ilex. The yellow looks like ill health to me, rather than variegation.

If the yellow traces back to the same branch, I'd assume it's variegation from a spontaneous mutation. (And as larry_gene said, pruning it out is an option if you don't like the look.)

The pattern of "some variegated / some not variegated" wouldn't look unusual on my variegated Euonymus -- but I have no experience with variegated Buxus.

Looks like the onset of phytophthora to me. The patterning of the yellowing leaves does not resemble a sport or mutation but the leaves simply dying. I'll bet that the symptoms progress over the upcoming weeks/months, as systemic diseases are wont to do. Phytophthora is very common to Buxus.