Lnk domains and Y537 contribute differently to the expansion of myeloid and megakaryocytic progenitors. (A) Schematic representation of WT and Lnk mutant forms cloned into the MIG retroviral vector. Point mutations in the PH domain (W270A), SH2 domain (R364M), and in the C-terminal tyrosine (Y536F) are indicated with an asterisk (*) or F. Total RNA was extracted from WT (+/+) or Lnk^−/−Lin⁻GFP⁺ progenitor cells transduced with either vector alone, WT, or mutant forms of Lnk, and WT APS and subjected to RT-PCR with specific primers for Lnk and APS (top panels) or HPRT (bottom panels) as control. (B-C) WT or Lnk^−/−Lin⁻GFP⁺ transduced cells were assessed for their in vitro colony-forming ability of multilineage (B) or megakaryocytic (C) progenitors in methylcellulose media containing optimal concentrations of appropriate recombinant growth factors. Data represent the mean ± SD (error bars) of number of colonies/10³ (for CFU-GEMM) or 1.5 × 10⁴ (for CFU-Meg) Lin⁻GFP⁺ cells from triplicate samples from 3 independent assays using 3-5 mice of each genotype. Statistical significance was determined using the Student t test: *P ≤ .002; **P ≤ .02; ***P ≤ .01. CFU-GEMM, colony-forming unit of granulocyte, erythroid, macrophage, and megakaryocyte cells; CFU-Meg, colony-forming unit of megakaryocyte cells.