p21-antisense increases primitive colony-forming cells without altering total colony number in CFC assays. CD34⁺ or CD34⁺38^– cord blood cells were transduced with p21-antisense or the control vector. After transduction the cells were plated in semisolid, cytokine-containing CFC medium. Colonies were scored after 10 days. (A) Mean ± SEM of total colonies per 600 transduced cells (n = 4). (B) Mean ± SEM colonies containing mixed red and white hematopoietic cells (CFU-mix) per 600 transduced cells (n = 4).