Fragile X expression in short-term whole blood cultures is affected by cell density

The effect of cell density on expression of the fragile site at (X)(q27.3) in short‐term whole‐blood cultures from patients with fragile X [fra(X)] or Martin‐Bell syndrome was studied. A significant increase in fra(X) frequency was observed in 7 of 8 samples when cell density was decreased. Higher fra(X) frequency was not always noted at below‐routine density, but in some cases fra(X) expression was depressed at above‐routine density. We conclude that decay of the FUdR effect explains the fact that fra(X) expression is affected by culture density. It is significant that a relationship exists between the two; it suggests that in order to maximize fra(X) expression in cases with low‐percentage fra(X) with standard methods, cell density may have to be adjusted. It is possible that in individuals who are normally nonexpressing, such as some obligate female carriers and nonpenetrant males, fra(X) expression may be sensitive to cell density effects.