Let's Get it on

So currently working with mice.  Doing research on breast cancer, so these mice are female with tumors growing in their mammary glands.  One set the cancer cells were injected into the glands.  The other set, the mice are genetically susceptible to developing cancer. 

So today we were injecting the treatment into the transgenic mice.  We put them to sleep so that they are easier to work with and so that it's also kinder to them.  We are about half way through and I hear some squeeking which draws my attention.  And it starts to become clear why one of the mice isn't developing any tumors.  I turn to the other lady and ask if she's sure that they are all female.  She claims they are, but we both start paying more attention.  It was then time to pick up this suspicious mouse.  And it was obvious that we had a Casanova in with our females.