Citation trends. (a)-(c) Scientific papers citing PCR, pBR322 cloning vector, and Maxam-Gilbert sequencing by year after initial publication. Citation counts were obtained by searching Web of Science citation databases by year for articles referencing the seminal publication for each technology. Counts begin with the first full year after publication. For PCR, the citation count includes references to both the 1985 Saiki paper and the 1987 Mullis paper. (d) Normalized citation data. Y-axis represents percent of total citations in a given year for each technology. X-axis in all graphs (a-d) represents years after initial publication.