I'm leaning towards female. Is it the runt of the litter? Often they don't develop nipple marks as quickly as their siblings. You likely won't know for sure until it's a little bit older though.

 

I am going to say it looks like a buck. Some bucks I've had look a bit different down there than others. Keep an eye on him as he matures, because if you separate him into the wrong group of sexes, you might have a big disaster.

 

i'm too torn to answer- let us know how it winds up, please?

 

They're now 16 days old and of course hopping around like mad so it's harder to tell, but I'm sure I saw nipples this time around. I didn't expect nipples to be obvious on a mouse of this age but they certainly looked that way to me. That combined with the anal-genital gap still being very small makes me more certain that she is definitely a she. Having said that I'm still keeping an eye on her just in case.

 

I'm thinking buck

 

Check for the vaginal opening - it's often easier to spot than nipples.

 

I'm also going with buck.