Erwin Chargaff was an Austro-Hungarian biochemist and a professor of biochemistry in Columbia University. He is known for his proposed rules that are referred to now as Chargaf's rules. Accordingly, a natural DNA would have the same number of guanine units and cytosine units. Likewise, the number of adenine units would equal the number of thymine units. Thus, this would implicate base pairing in DNA. This rule of Chargaff disproved the formerly accepted hypothesis called tetranucleotide hypothesis. In tetranucleotide hypothesis, it stated that DNA was comprised of a number of repeats of guanine, adenine, cytosine, and thymine, and variations in the equimolar base ratios were due to experimental error. Chargaff refuted it and proved them wrong through his experimentation that made use of paper chromatography and ultraviolet spectrophotometer. He later met with Francis Crick and James Watson to explain what he found in his research. Many believed that this first rule of Chargaff helped the research team of Watson and Crick to conclude the double helix structure of DNA. The second rule is that the amounts of adenine, thymine, guanine, and cytosine would vary from one species to another, implicating that the genetic material is DNA rather than protein.