THE KNOTTED class of plant genes encodes homeodomain proteins(1,2). These genes have been found in all plant species where they have been sought(3-5) and, where examined, show expression patterns that suggest they play an important role in shoot meristem function(5-7). Until now, all mutant phenotypes associated with these genes have been due to gain-of-function mutations(4,5,8,9), making it difficult to deduce their wild-type function. Here we present evidence that the Arabidopsis SHOOT-MERISTEMLESS (STM) gene, required for shoot apical meristem formation during embryogenesis, encodes a class I KNOTTED-like protein. We also describe the expression pattern of this gene in the wild-type plant. To our knowledge, STM is the first gene shown to mark a specific pattern element in the developing plant embryo both phenotypically and molecularly.