The descriptive designation, alveolar soft-part sarcoma, continues to
be used for this uncommon soft-tissue malignancy because an acceptable
hypothesis for its histogenesis has not been advanced, despite studie
s with electron microscopy and immunohistochemistry. These techniques
have, nevertheless, provided significant information that is useful in
the differential diagnosis of the tumor and pertinent in speculation
concerning its nature. The most intriguing ultrastructural feature is
the secretory process that culminates in the formation of the distinct
ive cytoplasmic crystals. Myogenic differentiation has been favored in
a number of recent reports on the basis of immunohistochemical findin
gs and the presence of the crystals does not rule out the possibility,
but accounts of immunoreactivity for the myogenic regulatory protein
MyoD1 have not been confirmed in subsequent studies or in the authors'
own staining of six cases.