Constitutive expression of RET transcripts and GDNFR-α in leukemic cells of myeloid origin (left panels) and in malignant cells from lymphoid tumors (right panels), as assessed by RT-PCR. One microgram of total RNA from AML cases (identified according to their FAB classification) and various lymphoid tumors was reverse-transcribed and amplified with primers specific for RET and GDNFR-α. PCR products were resolved on agarose gel, blotted, and hybridized with specific RET and GDNFR-α oligoprobes. An adult human brain substantia nigra cDNA and cDNA derived from the MN-60 cell line were used, respectively, as positive (+) and negative (−) controls for RET and GDNFR-α expression. cDNAs were always tested with β-actin–specific primers.