Alternative splicing and selective transport of RNA transcripts from cell nuclei are important regulatory mechanisms of gene expression during embryonic development. Here we report the molecular characterization and developmental expression in several tissue and organ systems of chicken hnRNP A1, a nucleo-cytoplasmic 'shuttle' protein which in mammalian systems has been shown to function in the regulation of RNA alternative splicing by antagonizing constitutive splicing factors such as SF2/ASF. We show that hnRNP A1 is represented in the chicken by a single gene which is widely expressed at early embryonic stages, with particularly high levels of expression in the brain, skin, developing gut, and other ectodermal and endodermal derivatives. At later stages, expression of its mRNA and protein product become progressively confined to specific organ primordia and cell types, where both transient and persistent expression patterns are observed. HnRNP A1 protein is expressed at sites of active neurogenesis in the developing central and peripheral nervous systems, regions of known extensive alternative splicing.