For many years it has been known that some breast cancers contain proteins called steroid receptors on the cells.

The two main steroid receptors are oestrogen receptors (ER) and progesterone receptors (PgR). Tumours that contain receptors are called ER+ve (oestrogen receptor positive) and PgR+ve (progesterone receptor positive).

Usually most, but not all, cancers which are ER+ve will also be PgR+ve. Similarly most, but not all, cancers which don't have ER (oestrogen receptor negative, ER-ve tumours) will not have PgR receptors (progesterone receptor negative, PgR-ve tumours).

The main importance of steroid receptors is to help in deciding on the right treatment. If a breast cancer is ER+ve then it is much more likely to respond to hormone treatments than a tumour that is ER-ve.

Putting figures to this, overall about 50% of breast cancers that are ER+ve will respond to hormone treatments but if the cancer is ER-ve then less than 1 in 10 will benefit.

Progesterone receptors are less important than oestrogen receptors in predicting the likely response to hormone treatment but they allow some fine tuning of these figures. Someone who has a breast cancer that is ER+ve and PgR+ve has about a 60% chance of benefiting from hormone therapy, whereas if the tumour is ER+ve but PgR-ve then this figure falls to about 40%. Similarly if both ER and PgR receptors are negative then the chance of a benefit from hormone treatment is less than 1 in 20.

Although the presence or absence of PgR slightly alters the chances of a response to hormone treatment it usually would not influence what was actually done - it is very unlikely that anyone whose cancer was ER+ve would not be offered hormone therapy simply because tests showed it was also PgR-ve. In practical terms, the presence or absence of oestrogen receptors is much more important than progesterone receptors.