(A) RT-PCR amplification of marrow stromal RNA using primers complementary to sequences on each side of the alternatively spliced IIICS region of fibronectin. Ten microliters of PCR products was run out on a TBE/2% agarose gel. Lane 1, reaction with RT; lane 2, control without reverse transcriptase; lane 3, DNA marker. The four bands in lane 1 are consistent with variants V120 (464 bp), V95 or 89 (389 or 371 bp), V64 (296 bp), and V0 (104 bp). (B) Restriction enzyme digests of selected clones prepared from TA cloning RT-PCR products of IIICS region of fibronectin. Three microliters of digested products was run on a TBE/1% agarose gel.BglII cuts the IIICS region when the CS1 sequence is present. A second BglII site in the pCR^TM II vector results in 1,155- and 3,148-bp products in the presence of a CS1/IIICS-B insert (V89) as shown in lanes 2 and 4. The band in lane 3 is consistent with a IIICS-B insert (V64) within the linearized vector (4,124 bp).