White Spot in Eye of (UK) Blacks

This white spot on the eye of a black mouse popped up recently on the Fancy Mice UK group. If you see these spots on the eye(s) of your blacks, FEAR NOT. It is apparently totally normal and is described as "unavoidable". It is more common in some lines than others, and does not hurt the animal in any way. They also come and go with little or no known cause, so there is no reason to cull the animals in question. The mouse shown here is owned by Kelly Jeffrey, who gave permission for me to repost her photo to explain the issue described above. Please do not repost the photo without her name and a clear explanation that this is common in UK blacks and not harmful.

Curious! What is it and what causes it?

We don't know and we don't know.
If somebody can figure out what the Brits can't about their mice, well...

Well, have they eliminated cataracts?

Cataracts don't expand and contract like a pupil. They are on the surface of the lens, rather than behind it. They also don't tend to come and go overnight like these do.

Wow!