An intracellular timer in oligodendrocyte precursor cells is thought to help control the timing
of their differentiation. We show here that the expression of the Hes5 and Mash1 genes,
which encode neural-specific bHLH proteins, decrease and increase, respectively, in these
cells with a time course expected if the proteins are part of the timer. We show that enforced
expression of Hes5 in purified precursor cells strongly inhibits the normal increase in the
thyroid hormone receptor protein TR β 1, which is thought to be part of the timing …

An intracellular timer is thought to help control the timing of oligodendrocyte differentiation.
We show here that the expression of the helix–loop–helix gene Id4 in oligodendrocyte
precursor cells decreases in vivo and in vitro with a time course expected if Id4 is part of the
timer. We also show that Id4 expression decreases prematurely when the precursor cells are
induced to differentiate by mitogen withdrawal. Both Id4 mRNA and protein decrease
together under all of these conditions, suggesting that the control of Id4 expression is …