(A) Activated NK cells express message encoding the standard CD44 isoform. RNA from unactivated (lane 1) and IL-2–activated (lane 2) LD-PBL and from MDMB-75 breast cancer cells (lane 3) was analyzed by RT-PCR using primers flanking the CD44 variant domain insertion point. Amplified products were detected by Southern blotting. Positions of standard and variant isoforms are indicated. (B) Unactivated and activated NK cells express similar sized forms of CD44 on their surfaces. Immunoprecipitates from biotin-labeled purified NK cells were analyzed by SDS-PAGE and Western-blotted with streptavidin-HRP. The cells were either unactivated (lanes 1 and 2) or activated (lanes 3 and 4). Anti-CD44 MoAb was used for immunoprecipitation in lanes 1 and 3, and an isotype-matched control antibody (MOPC 300) was used in lanes 2 and 4.