Description on en.wiki:

David Ruben created this image, modifying existing images of various tetracyclines to show core 4 rings. Hence tetra- (4) -cycl- (ring) -ine (standard suffix for antibiotics)

I've only marked out consistent attached groups. The top right oxygen seems always present as either a '=O' or '-OH' group (hence the faded out 'H'). All other possible locations used for a variety of attached groups are marked with '—', but note that not all of these are utilised in any one particular member of the Tetracycline antibiotics group.

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