There was an error published in Development 132, .
The sentence on page 4550 ‘Following Berg and Purcell (Berg and Purcell, 1977), threshold detection of the gradient occurs when: 
where δbin is the uncertainty in the concentration measurement for one bin.’
should instead have read:
‘Following Berg and Purcell (Berg and Purcell, 1977), threshold detection of the gradient occurs when: 
where δbin is the uncertainty in the concentration measurement for one bin.’
The publishers apologise to the authors and readers for this mistake.
