Colony formation by MTHC infected with retroviruses carrying human Kit encoding constructs. The parent MTHC infected with retroviruses containing either RUFneo, c-kitRUFneo, or mutant kitRUFneo are represented as Vector, WTKit, and V816Kit, respectively. After 1 week of G418 selection in the presence of GM-CSF, cells were plated in triplicate (20,000 per dish) in methylcellulose containing either murine GM-CSF (▪) or no factor (□). Colonies containing more than 50 cells were scored 1 week after plating and the mean colony numbers ± the SEM are shown with the asterisk (*) indicating an absence of colonies.