Pigpen is a 67-kDa Sepharose-binding molecule isolated from mammalian endothelial and retinal pigmented epithelial cells. The protein is distributed nonhomogeneously in the nucleus, exhibiting diffuse staining throughout (excluding nucleoli), together with a small number of intensely stained focal points, or granules, and punctate staining along the nuclear envelope. Pigpen was absent or greatly attenuated in the nonepithelial cell types we examined, including fibroblasts, myeloma, and astroglia. cDNA sequence analysis revealed a positively charged molecule with an RNP-CS RNA-binding domain, 19 RGG repeats, and a consensus tyrosine phosphorylation site in the C-terminus. The amino terminal portion of the molecule is characterized by 7 glutamine-rich hexapeptide repeats similar to those found in the transactivation domain of known transcription activators. Pigpen has a high level of identity with the FUS gene product, TLS (Translocated in Liposarcoma; Crozat et al, 1993; Rabbits et al., 1993), a new member of the EWS family of proteins. Expression of pigpen is regulated during the transition between active and quiescent endothelial cell phenotypes. Both mRNA and overall protein levels are maintained at a steady level in actively growing cells. The number of nuclear granules increases as cultures approach confluency. When cells reach confluency, overall expression is sharply reduced and the number of nuclear focal points declines gradually. We observed that reactivation of endothelial cells locally by wounding of confluent cultures resulted in a spatially restricted reactivation of pigpen expression. This pattern of expression, taken together with structural data, suggests that pigpen may function in the growth and differentiation of endothelial cells during angiogenesis.