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PG 0043+039 is the most X-ray weak quasar known to date. Marginally detected in 2005 comma an XMM- Newton observation in 2013 set a clear detection im- plying a flux increase of at least a factor 3.8. Simulta- neous observations at other wavelengths showed the quasar still extreme X-ray weak comma with a peculiar spec- tral energy distribution (SED) and unique features in the UV spectrum. We propose that PG 0043+039 is a genuine X-ray weak quasar. To probe this hy- pothesis and to test alternative emission scenarios (ADAFs comma missing corona (Ricci et al. comma 2020 ApJ 898 comma L1) comma cyclotron-emission (Kollatschny et al. 2015 comma A and A comma 577 comma L1)) this unique source deserves further comma deeper comma observations comma aiming to establish its X-ray spectrum comma its optical to hard X-ray SED and new con- straints on its variability. Our proposal is to observe PG 0043+039 quasi-simultaneously (2 days) with XMM-Newton (100ks + 20ks) comma NuSTAR (60ks) and HST (2 orbits). We will arrange quasi-simultaneous observations with the ground 10m Hobby-Eberly Telescope in Texas and with the 10m SALT Telescope in South Africa.