A Theorem of Gao, Jackson and Seward, originally conjectured to be false by Glasner and Uspenskij, asserts that every countable group admits a strongly aperiodic subshift over a 2-symbol alphabet. Their proof consists of a quite technical construction. We give a shorter proof of their result by using the asymetrical version of Lovasz Local Lemma which allows us also to prove that this subshift is effectively closed (with an oracle to the word problem of the group) in the case of a finitely generated group. This is about joint work with Nathalie Aubrun and Stéphan Thomassé.