1. PubChem presents users with the complexity of parallel systems of D-C connectivity . For Medical Subject Headings (MeSH) the publication links are biased towards common name matches in many papers (e.g. the MeSH category for Chemicals and Drugs links 127K PubChem CIDs to over 14 million PMIDs). Somewhat surprisingly, the largest D-C source by far is the IBM automated patent extraction system. This has operated on not only patents but also PubMed (plus MeSH terms in those abstracts) as well as full text from PMC articles. By 2016 this was responsible for 56% of all PMID-CID mappings (although IBM made what may have been their final submission in 2017). PubChem has a third substantial category of D-C connectivity from the publishers Springer, Theime and most recently Wiley. These three sources have added document links for 660, 740 and 118 thousand CIDs respectively (with an overlap of only 74K). However, those having DOI-only document links are not connected into Entrez. They are made accessible via cross-references in the CIDs for Theime and Wiley but only via SIDs for Springer. Since these publishers have provided proof of concept for these D-C efforts it is to be hoped that they will be followed by equivalent undertakings from other chemistry publishers, for example ACS, Elsevier and ChemRxiv.