The role of preprints in the scientific production and their part in citations have been growing over the past 10 years. In this paper we propose to study the citations of preprints in several different aspects: the progression of these citations over time, the sections of the articles in which they are most likely to appear and the percentage of such citations with respect to all citations in articles. We have processed the PLOS dataset that covers 7 journals and a total of about 240000 articles up to January 2021. The results show that preprint citations are found with the highest frequency in the Method section of articles, though small variations exist with respect to journals. The PLOS Computational Biology journal stand out as it contains more than three times more preprint citations than any other PLOS journal. The relative parts of the different preprint databases are also examined. While ArXiv and bioRxiv are the most frequent citation sources, bioRxiv’s disciplinary nature can be observed as it is the source of more than 70% of preprint citations in PLOS Biology, PLOS Genetics and PLOS Pathogens.