Isolation of immortal CD56+ clones. (A) FACS histograms of primary 01Abic cells at PD7 (left) and two clones at PD35, one CD56+ and the other CD56-, after immortalization with CDK4 and human telomerase reverse transcriptase (hTERT) (right). Red lines represent cells treated with anti-CD56 antibody; green lines represent controls where the primary antibody was omitted, and hence they correspond to nonspecific background fluorescence. The percentage of CD56+ cells (fluorescence greater than background) is indicated. (B) Telomere restriction fragment analysis gel showing telomere length dynamics of the CD56+ population and two immortalized CD56+ clones at different PDs. (C) Growth curves of primary 01Abic cells sorted for CD56 (CD56+ population, red squares), cells infected with CDK4 (CD56+ population CDK4, red diamonds) and three CD56+ immortalized clones (blue lines).