The present invention relates to method of enriching NKX6.1 and C-peptide co-expressing cell aggregates derived in vitro from stem cells said method comprising the steps of dissociating the endocrine cell aggregates into single cells, treating the single cells with cryopreservation medium and lowering temperature to obtain cryopreserved cells, thawing the cryopreserved cells; and re-aggregating the cells obtained after thawing into endocrine cells.