Two isoforms of the src-family tyrosine kinase p59fyn have been shown to arise through alternative splicing of exon 7 of the fyn gene. These isoforms have been designated fynT, expressed in hematopoietic cells, and fynB, expressed in the brain. Here, we describe a novel isoform, fynDelta7, in which exon 7 is absent. FynDelta7 mRNA transcripts have been identified in peripheral blood mononuclear cells from all individuals tested thus far and semi-quantitative RT-PCR indicates that this fyn transcript is expressed at near wild-type levels. Transcripts coding for the fynDelta7 isoform can be detected in purified B cells, T cells, NK cells, and monocytes, indicating that it is not lineage specific. We further show that the message isolated is functional using an in vitro expression system and by its expression in COS cells. (C) 2002 Elsevier Science (USA). All rights reserved.