Mulberry tree over-growing

Hi

I've had a mulberry tree for 8 or 9 years, and suddenly it has grown a very tall new stem out of the top.

This is the first year this has happened- I pruned it as normal (about a month after leaf fall) and it has lots of mulberries as usual.

I'd appreciate if anyone could tell me what is happening and whether I should cut this new stem off or not.

Thank you!

 

Thanks for your reply!
The tree has been grafted- is it more likely that this new shoot is from the tree the mulberry branches were grafted to?
The leaves on the new shoot look slightly different to the ordinary mulberry leaves...