My-bi HSCs contribute significantly longer to peripheral white blood cells than do other types of HSCs. Shown are the percentages of donor-type cells in blood measured at the time points indicated by crosses for each of the HSC clones. Clonally derived HSCs were classified as My-bi (A), Ly-bi (B), and balanced (C) according to the lymphoid-to-myeloid cell ratios in blood generated in the primary hosts. Early repopulation patterns of some of the clones have been reported previously.¹⁰  Primary hosts were rested at least 6 months, and BMCs from these mice were then transplanted into secondary hosts at the time points indicated by arrows. Whenever the secondary hosts showed good levels of repopulation at 7 months after transfer, BMCs were transplanted into tertiary and, if possible, into quaternary hosts at the time points indicated by the arrows. The time of injection into the first host is called month 0, and the cumulative number of months that an HSC clone contributed donor-type cells is shown on the x-axis. Multiple hosts were used in serial transplantation, and the mean levels of donor-type cells in blood are shown. As reported previously, ¹⁰ the extent of repopulation in multiple recipients of clonally derived HSCs is similar. Some HSC clones (indicated by asterisks) are still contributing to donor-type cells. Clones that exhausted and failed to self-renew in the primary host (and thus could not reconstitute secondary hosts) were classified as graft failures (D).