with the following code, xlab will be bold but ylab not. could you please help me

pdf()

par(font.axis=2, font.lab=2, cex.lab=1.2, cex.axis=1.2, cex.sub=1.2, mfrow=c(2,2), mar = c(5,6,3,1) + 0.1)

hist (ADAM, axes=F, main="", ylim=c(0,1000000), xlab="", ylab= expression(paste("number(",x10^6,")",sep="")))

axis(1,at=seq(0,1,0.2), labels=seq(0,1,0.2))

axis(2, at=seq(0,1000000,500000), labels=seq(0,1, by=0.5), las=2)