Immortalized Bax-null myogenic cells had normal chromosome numbers. Chromosome numbers of individual cells were determined in (i) primary cultures of two independent, early passage number <4, wild-type myogenic cells (n = 66); (ii) late passage cultures of two independent, immortalized, Bax-null myogenic lines (n = 49); and (iii) a late passage culture of one line of immortalized p53-null myogenic cells (n = 32). (In this and subsequent figures, late passage, immortalized lines were examined at passage numbers 17 – 26 corresponding to >60 days of total culture duration; and early passage, non-immortalized lines were examined at passage numbers < 4 corresponding to less than 12 days of total culture duration and prior to the slow growth period.) Pooled results are shown for the two wild-type and the two Bax-null lines (BaxKO lines #1 and #2, see also Figs. 4 and 5), because, in each case, the two lines had statistically identical chromosome numbers. Both wild-type and Bax-null myogenic cells had normal karyotypes with approximately 40 chromosomes, whereas p53-null myogenic lines had approximately twice the normal number of chromosomes.