ends present in linear RNA molecules have been joined. Sanger et al. also provided evidence for the true circularity of viroids by finding that the RNA could not be phosphorylated at the 5' terminus. In other tests, they failed to find even one free 3' end, which ruled out the possibility of the molecule having two 3' ends. Viroids thus are true circular RNAs. The single-strandedness and circularity of viroids was confirmed by electron microscopy, The complete nucleotide sequence of potato spindle tuber viroid was determined in 1978. PSTVd was the first pathogen of a eukaryotic organism for which the complete molecular structure has been established. Over thirty plant diseases have since been identified as viroid-, not virus-caused, as had been assumed. Four additional viroids or viroid-like RNA particles were discovered between 2009 and 2015. In 2014, New York Times science writer Carl Zimmer published a popularized piece that mistakenly credited Flores et al. with the hypothesis' original conception. See also References External links Viroids/ATSU ViroidDB, a database of viroids and viroid-like circular RNAs