In 1994, one of fellow graduate students at South Dakota State gave me the moniker "DNADave". Fast forward seven years and I was teaching a course in the Biostatistics Department at the University of Washington. The IT guy in the department asked for a list of nicknames that could be used for my email address and DNADave was one of 11 I submitted. He chose that one. Since then, I've used that nickname as my username whenever and wherever I could.
As to why DNADave? I am a statistical geneticist doing research in both animal and human genetics and genomics.