Experimental studies reveal that double-stranded DNA can exist in three different helical forms known as A, B, and Z DNA. Why do molecular biologists primarily focus on B DNA? B DNA is the only one of the three forms that is a right-handed helix. B DNA was discovered first, and has been studied longer than the other two forms. The structure of the B DNA is least affected by interactions with other molecules. B DNA is the simplest of the three structures, and the easiest to model mathematically. B DNA is a good approximation of the structure of DNA in cells.