The MS lineage of C. elegans produces three distinct muscle types. The 2 sex myoblasts divide and form the 8 uterine and 8 vulval muscles. The first allele of sem-2 was identified as an Egl mutant. From polarized light microscopy it was determined that the Egl phenotype was caused by the absence of the sex muscles. 1 of the newly identified alleles results in a phenotype similar to the original sem-2 mutation.
Mutant embryos fail to elongate their heads resulting in a bloated head appearance.
Further antibody staining showed however that in the mutant embryos the MS lineage is still capable of producing pharyngeal muscles. Sulston, J. E. and H. R. Horvitz.