Let D be an open subset of a two-dimensional Stein manifold S. Then D is Stein if and only if every holomorphic line bundle L on D is the line bundle associated to some (not necessarily effective) Cartier divisor 𝔡 on D.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-ap83-3-8, author = {Makoto Abe}, title = {Holomorphic line bundles on a domain of a two-dimensional Stein manifold}, journal = {Annales Polonici Mathematici}, volume = {83}, year = {2004}, pages = {269-272}, zbl = {1103.32010}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-ap83-3-8} }
Makoto Abe. Holomorphic line bundles on a domain of a two-dimensional Stein manifold. Annales Polonici Mathematici, Tome 83 (2004) pp. 269-272. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-ap83-3-8/