p21-antisense increases the frequency of long-term primitive culture-initiating cells (LTC-ICs) in transduced CD34⁺ and CD34⁺38^– cord blood cells. Transduced CD34⁺ and CD34⁺38^– cord blood cells were plated on irradiated human stroma in limiting dilutions. After 5 weeks the medium was replaced with semisolid CFC medium. Frequency of LTC-ICs in p21-antisense (p21-AS-V)– and control vector (GFP-V)–transduced CD34⁺ (A) or CD34⁺38^– (B) cord blood cells (n = 3 CD34⁺ cells, n = 5 CD34⁺38^– cells; dotted line represents average) is shown.