  Current observations favour that the massive ultraviolet-bright clumps with a
median stellar mass of $\sim 10^7~M_{\odot}$, ubiquitously observed in $z\sim
1-3$ galaxies, are star-forming regions formed in-situ in galaxies. It has been
proposed that they result from gas fragmentation due to gravitational
instability of gas-rich, turbulent, high-redshift discs. We bring support to
this scenario by reporting the new discovery of giant molecular clouds (GMCs)
in the strongly lensed, clumpy, main-sequence galaxy, A521-sys1, at $z=1.043$.
Its CO(4-3) emission was mapped with the Atacama Large Millimeter/submillimeter
Array (ALMA) at an angular resolution of $0.19''\times 0.16''$, reading down to
30~pc thanks to gravitational lensing. We identified 14 GMCs, most being
virialized, with $10^{5.9}- 10^{7.9}~M_{\odot}$ masses and a median
$800~M_{\odot}~\rm{pc}^{-2}$ molecular gas mass surface density, that are,
respectively, 100 and 10 times higher than for local GMCs. They are also
characterized by 10 times higher supersonic turbulence with a median Mach
number of 60. They end up to fall above the Larson scaling relations, similarly
to the GMCs in another clumpy $z\simeq 1$ galaxy, the Cosmic Snake, although
noteworthy differences between the two sets of high-redshift GMCs exist.
Altogether they support that GMCs form with properties that adjust to the
ambient interstellar medium conditions prevalent in the host galaxy whatever
its redshift. The detected A521-sys1 GMCs are massive enough to be the parent
gas clouds of stellar clumps, with a relatively high star-formation efficiency
per free-fall time of $\sim 11$ per cent.
