International entrepreneurship (IE) research has grown rapidly, encompassing many industries and world regions. Past IE research has examined the macro, industry and firm-specific variables that lead to companies' early internationalization and its financial and non-financial outcomes. Most prior IE research has been correlational in focus and static in design. Focusing on early internationalization, we propose that a significant shift can occur in IE research by applying a cognitive perspective and examining how entrepreneurs recognize and exploit opportunities in international markets. A cognitive approach will allow researchers to probe entrepreneurs' motivations to internationalize and capture their mental models. The article highlights the benefits to be gained from and the challenges associated with using a cognitive approach to IE research.

CD123 (␣-subunit of IL-3 receptor) expression on primitive and committed human hematopoietic cells was studied by multicolor sorting and single-cell culture. The sources of cells included fetal liver (FLV), fetal bone marrow, umbilical cord blood, adult bone marrow and mobilized peripheral blood. Three subsets of CD34 + cells were defined by the levels of surface CD123: CD123 negative , CD123 low , and CD123 bright . Coexpression of lineage markers showed that a majority of CD34 + CD123 bright cells were myeloid and B-lymphoid progenitors, while erythroid progenitors were mainly in the CD34 + CD123 negative subset. The CD34 + CD123 low subset contained a heterogeneous distribution of early and committed progenitor cells. Single CD34 + cells from the CD123 subsets were cultured in a cytokine cocktail of stem cell factor, interleukin 3 (IL-3), IL-6, GM-CSF, erythropoietin, insulin-like growth factor-1, and basic fibroblast growth factor. After 14 days of incubation, a higher cloning efficiency (CE) was observed in the CD34 + CD123 negative and CD34 + CD123 low fractions (37 ± 23% and 44 ± 23%, respectively) than in the CD34 + CD123 bright fraction (15 ± 21%). Using previously published criteria that colonies containing dispersed, translucent cells (dispersed growth pattern, DGP) were derived from primitive cells and that colonies composed solely of clusters were from committed cells, early precursors were distributed evenly in the CD34 + CD123 negative and CD34 + CD123 low subsets. When CD38 and CD90 (Thy-1) were used for further characterization of CD34 + cells from FLV, CE increased from 37 ± 23% in CD123 negative to 70 ± 19% in CD123 negative CD38 -and from 44 ± 23% in CD123 low to 66 ± 19% in CD123 low CD38 -. No significant increase in CE or DGP progenitors was observed when CD34 + cells were sorted by CD90 and CD123. We concluded that: A) high levels of CD123 were expressed on B-lymphoid and myeloid progenitors; B) early erythroid progenitors had little or no surface CD123, and C) primitive hematopoietic cells are characterized by CD123 negative/low expression.

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