In this work, we analyzed the phenotype and growth of human NK cell clones obtained by the stimulation of individual NK cells with IL-2 and gene-modified K562 feeder cells expressing membrane-bound IL-21 (K562-mbIL21). We generated clones from NK cells at distinct differentiation and activation stages, determined by CD56, CD57 and HLA-DR expression levels. Less differentiated CD56bright NK cell subsets showed higher cloning efficiency compared with more differentiated CD56dim subsets, especially with the CD57bright subset. However, clones from the CD56dimCD57- subset lived longer on average than other subsets. Moreover, several clones with the highest cell numbers were derived from CD56dimCD57-HLA-DR-cells. Most of the clones including those derived from more differentiated CD56dimCD57+/-NKG2A- NK cells showed a less-differentiated NKG2A+ phenotype. Also, CD57- cells were frequently observed in clones derived from CD57+ NK cells suggesting the loss of CD57 during the cloning process. On the other hand, KIR surface expression once detected for a clone never disappeared entirely, confirming irreversibility of the KIR expression. In summary, we have demonstrated that in specific conditions terminally differentiated CD57+ human NK cells are able to acquire the CD57- phenotype that was previously not observed and, thus, was considered impossible.