De Arce MA, Costigan C, Gosden JR, Lawler M, Humphries P. Further evidence consistent with Yqh as an indicator of risk of gonadal blastoma in Y‐bearing mosaic Turner syndrome. Clin Genet 1992:41:28–32.An 8‐year‐old girl with some features of Turner syndrome and karyotype 45X/46XY had developed a bilateral gonadoblastoma in her rudimentary ovaries. Her normal Y chromosome showed the characteristic distal fluorescence, as seen in her father's. Another mosaic, this time 45X/46XidicY, and also with some Turner features had rudimentary ovaries, but no gonadoblastoma had developed at age 14.The nature of her idicY, which showed no fluorescent distal Yq and had one of the centromeres inactivated, was confirmed byin situhybridisation with a Yp‐specific probe. Using primers from a human Yp‐specific sequence, we amplified DNA extracted from paraffin‐embedded ovarian tissue from both cases, and from a normal testicle and a normal ovary as controls. The finding of the expected Y‐derived PCR product in the rudimentary gonads from these mosaic patients indicates the presence of their Y chromosome in both. We discuss the validity of the findings, and the possible role of sequences in or near the fluorescent part of Yq in the origin of gonadoblastoma in Y‐bearing mosai