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Many fungi are capable of growing by polarized cellular extension to form hyphae or by isotropic expansion to form buds. Aspergillus nidulans anucleate primary sterigmata (apsA) mutants are defective in nuclear distribution in both hyphae and in specialized, multicellular reproductive structures, called conidiophores. apsA mutations have a negligible effect on hyphal growth, unlike another class of nuclear distribution (nud) mutants. By contrast, they almost completely block entry of nuclei into primary buds, or sterigmata (bud nucleation), produced during development of conidiophores. Failure of the primary sterigmata to become nucleated results in developmental arrest and a failure to activate the transcriptional program associated with downstream developmental steps. However, occasionally in mutants a nucleus enters a primary bud and this event relieves the developmental blockage. Thus, there is a stringent developmental requirement for apsA function, but only at the stage of primary bud formation. apsA encodes a 183-kD coiled-coil protein with similarity to Saccharomyces cerevisiae NUM1p, required for nuclear migration in the budding process.